Historical linguistics has made tentative postulations about and multiple varyingly different reconstructions of Proto-Germanic grammar, as inherited from Proto-Indo-European grammar. All reconstructed forms are marked with an asterisk (*).
Overview
Proto-Germanic had six cases, three genders, two numbers (relics of dual survive in verbs and in some number words like 'two' or 'both'), three moods (indicative, subjunctive (PIE optative), imperative), and two voices (active and passive (PIE middle)). This is quite similar to the state of Latin, Greek, and Middle Indo-Aryan languages of . It is often asserted that the Germanic languages have a highly reduced system of inflections as compared with Greek, Latin, or Sanskrit. However, some parts of the inflectional systems of Greek, Latin, and Sanskrit were innovations that were not present in Proto-Indo-European.
Furthermore, it is probably due more to the late time of attestation of Germanic than to any inherent "simplicity" of the Germanic languages. As an example, there are fewer than 500 years between the Gothic Gospels of (see Ulfilas) and the Old High German Tatian of , yet Old High German, despite being the most archaic of the West Germanic languages, is missing a large number of archaic features present in Gothic, including dual and passive markings on verbs, reduplication in Class VII strong verb past tenses, the vocative case, and second-position (Wackernagel's Law) clitics. Many more archaic features may have been lost between the Proto-Germanic of or so and the attested Gothic language.
Furthermore, Proto-Romance and Middle Indo-Aryan of the contemporaneous with Gothichad significantly less inflexional complexity than Latin and Sanskrit, respectively, and overall features probably no more archaic than Gothic. At the same time, Middle Persian inflection had already been simplified to a stage perhaps comparable with modern Dutch.
Nouns The system of nominal declensions was largely inherited from PIE. Six cases were preserved: vocative, nominative, accusative, dative, instrumental, genitive. The instrumental and vocative can be reconstructed only in the singular. The instrumental survives only in the West Germanic languages, and the vocative only in Gothic. The locative case had merged into the dative case, and the ablative may have merged with either the genitive, dative or instrumental cases. However, sparse remnants of the earlier locative and ablative cases are visible in a few pronominal and adverbial forms, and in some instances the case forms of certain noun classes use the older locative ending for the dative.
The older distinction between athematic and thematic stems had been lost, and generally nouns were divided into several declension classes based on the vowels or consonants before the case endings. Globally, there were vowel stems (a-, ō-, i- and u-stems) and consonant stems (n-, r- and z-stems and stems ending in other consonants). Usually, only nouns ending in consonants other than n, r or z are called consonant stems in the context of Proto-Germanic nouns. The neuter nouns of all classes differed from the masculines and feminines in their nominative and accusative endings, which were alike.
a-stems The a-stems descended from the PIE thematic inflection and were by far the most common type of noun in Proto-Germanic. Although they could originally be any gender in PIE (as could be seen in Latin), in Proto-Germanic they were restricted to either masculine (ending in -az) or neuter (ending in -ą). The two genders differed only in the nominative, vocative and accusative cases; the other three cases were identical for both. There were two smaller subgroups within the a-stems, ja-stems, and wa-stems. These were declined the same as regular a-stems, but with a suffix -(i)j- or -w- before the ending. It was only in the daughter languages that the wa- and (especially) ja-stems began to diverge significantly from the regular a-stems.
The Proto-Indo-European genitive & dative singular endings *-osyo and *-oey, alongside the instrumental plural -ōys have the reconstructed results of *-as/-is, -ai, and *-amiz instead of the more expected **-az (same as the nominative singular *-az) and **-ōj (compare the nominative plural *-ōz from -oes) and -**ōiz. The reason for this is unclear, but some theories have been put forward as to why.
The irregular genitive could be use of a stressed and/or e-grade variant (**-ósyo or **-ésyo), which would give *-as or *-is instead of a more expected *-az or hypothetical *-iz (from **-esyo) after the loss of world final *-a -o -e -ya -yo -ye -wa -wo -we, as *-s would remain voiceless due to the accent falling on the suffix and not the root word. The best explanation for this irregular stressed variant being used for the genitive was probably the speakers of Germanic wanting to keep the nominative/genitive opposition intact in some form, finding it a useful distinction to make in speech.
The i-stem dative singular *-eyes and instrumental plural *-imis probably influenced the development of the a-stem endings too, giving Early Pre-Germanic **-oye and **-omis, which would give *-ai, and *-amiz after regular *a o > a merging, unstressed raising of *e to i, Verner's law and loss of intervocalic *-y-. The exact reason for these specific endings to be irregularly changed in analogy with the i-stem forms is unknown.
For the nominative and vocative plural, the northern West Germanic languages attest a voiceless *-s in the ending also. The origin of this is not clear.
*wulfaz ‘wolf’ (masc.) *juką ‘yoke’ (neut.) Singular Plural Singular Plural Nominative *wulfaz *wulfōz, -ōs *juką *jukō Vocative *wulf Accusative *wulfą *wulfanz Genitive *wulfas, -is *wulfǫ̂ *jukas, -is *jukǫ̂ Dative *wulfai *wulfamaz *jukai *jukamaz Instrumental *wulfō *wulfamiz *jukō *jukamiz
ō-stems The ō-stems descended from the thematic eh₂-stems in PIE, but there were also examples that descended from originally neuter collective nouns that were reinterpreted as feminine singulars. They were the feminine equivalent of the a-stems and were the most common type of feminine noun, with a nominative singular ending in -ō. There were also jō-stems and wō-stems, declined identically to the regular ō-stems but with a suffix before the ending.
*gebō ‘gift’ (fem.) Singular Plural Nominative-Vocative *gebō *gebôz Accusative *gebǭ *gebōz Genitive *gebōz *gebǫ̂ Dative *gebōi *gebōmaz Instrumental *gebō *gebōmiz
ī/jō-stems The ī/jō-stems descended from athematic/ablauting nouns in *-ih₂/yéh₂-. In Proto-Germanic, they had almost fallen together with the jō-stems, except that they had a nominative/vocative singular form in -ī rather than -jō. They did not survive as a distinct class in any language except possibly Gothic.
*bandī ‘fetter, bond’ (fem.) Singular Plural Nominative-Vocative *bandī *bandijôz Accusative *bandijǭ *bandijōz Genitive *bandijōz *bandijǫ̂ Dative *bandijōi *bandijōmaz Instrumental *bandijō *bandijōmiz
i-stems The i-stems descended from PIE nouns in -is, many of which were feminine abstract nouns in -tis. They were reasonably common and appeared in all three genders, although neuter i-stems were very rare with only a handful of reconstructible examples. The masculine and feminine i-stems were declined the same, with a nominative singular in -iz. The neuters ended in -i.
*gastiz ‘guest’ (masc.) *mari ‘sea’ (neut.) Singular Plural Singular Plural Nominative *gastiz *gastīz *mari *marī Vocative *gasti Accusative *gastį *gastinz Genitive *gastīz *gastijǫ̂ *marīz *marjǫ̂ Dative *gastī *gastimaz *marī *marimaz Instrumental *gastī *gastimiz *marī *marimiz
u-stems The u-stems descended from PIE nouns in -us, many of which were masculine nouns in -tus. They were formally parallel to the i-stems, but because of sound changes they had become a distinct class. They were less common than the i-stems overall and appeared in all three genders, although neuter u-stems were likewise very rare. The masculine and feminine u-stems were declined the same, with a nominative singular in -uz. The neuters ended in -u, but since there are no neuter plurals attested, their plural inflection can only be guessed.
*sunuz ‘son’ (masc.) *fehu ‘livestock’ (neut.) Singular Plural Singular Plural Nominative *sunuz *suniwiz *fehu (*-ū?) Vocative *sunu Accusative *sunų *sununz Genitive *sunauz *suniwǫ̂ *fehauz (*-iwǫ̂?) Dative *suniwi *sunumaz *fihiwi (*-umaz?) Instrumental *sunū *sunumiz *fehū (*-umiz?)
an-stems The an-stems were a common group of noun, descended from various kinds of PIE n-stem, all of which were athematic. They were either masculine or neuter, although neuters were rare. Their nominative singular forms ended in -ô. There were also jan-stems and wan-stems, which were declined mostly as regular an-stems. The an-stems correspond with Latin 3rd declension nouns such as homō (gen. hominis) "man" and nomen (gen. nominis) "name". They are also the source of many modern German weak nouns.
The masculine nominative singular ending cannot be reconstructed with confidence, as both North and East Germanic reflect a rather different ending. Old Norse -i and Gothic -a can conceivably come from an ending *-ē, but the source of such an ending is unknown. Kroonen also disputes the genitive singular ending, having the ending terminate in -az instead of -iz; for the sake of relevance, the -az genitive ending termination will be used in discussing declensions of zero-grade–ablauting an-stems in this article.
*gumô? ‘man’ (masc.) *augô? ‘eye’ (neut.) Singular Plural Singular Plural Nominative *gumô? *gumaniz *augô? *augōnō Vocative *gumô? Accusative *gumanų *gumanunz Genitive *guminiz *gumanǫ̂ *auginiz *auganǫ̂ Dative *gumini *gumammaz *augini *augammaz Instrumental *guminē? *gumammiz *auginē? *augammiz
Some nouns, such as *namô "name", preserved an alternate ablaut pattern, with the suffix in the zero grade in some forms.
*namô? ‘name’ (neut.) Singular Plural Nominative-Vocative-Accusative *namô? *namnō Genitive *naminiz *namnǫ̂ Dative *namini *namnamaz Instrumental *naminē? *namnamiz
Under the controversial Kluge's Law, a paradigmatic n in an an-stem immediately succeeding a stop consonant may fuse to create a geminate stop, in the same context as Verner's Law, seen most prominently in the genitive singular. The dative singular of ablauting an-stems would also be affected by Verner's law itself, causing the voicing of an originally voiceless root-final consonant. This voiceless geminate may then be reduced to a single voiceless stop after a heavy syllable. Furthermore, some an-stems contained root ablaut across their paradigms. All these morphophonological alternations would create multiple stems for a single noun's declension paradigm. The descendants of these ablauting an-stems in the daughter languages were derived from one morpohonological alternant generalized across a paradigm; paradigmatically leveled derivatives would be generally derived from the nominative singular stem, genitive singular stem, dative singular stem, accusative plural stem, or some chimera of these.
Kroonen reconstructs the following masculine an-stems incorporating Kluge's Law (of which he is an ardent supporter) and usually assuming the amphikinetic accent pattern.
*krebô? ‘basket’ *skinkô? ‘shank’ Singular Plural Singular Plural Nominative-Vocative *krebô? *krebaniz *skinkô? *skinkaniz Accusative *krebanų *kurpunz *skinkanų *skankunz Genitive *kurpaz *kurpǫ̂ *skunkaz *skunkǫ̂ Dative *krebini *krebummaz *skungini *skungummaz Instrumental *kurpē? *krebummiz *skunkē? *skungammiz