In all but the third conjugation, only the thematical vowel of the stem is used.
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Certain verbs change their stem vowel for the second and third person singular forms.
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In modal verbs, the stem vowel will change for all conjugations of the singular simple present.
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This happens when the stem vowel receives the stress.
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Regular strong verbs were all conjugated roughly the same, with the main differences being in the stem vowel.
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In Indo-European linguistics, a thematic stem or vowel stem is a noun or verb stem that ends in a vowel that appears in or otherwise influences the noun or verb's inflectional paradigm.
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In Greek and Latin grammar, combining bases usually require a thematic or stem-forming vowel.
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In nouns with two stems, the stem with the long vowel is called the strong stem, while the stem with the short vowel is called the weak stem.
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A considerable number of verbs change the vowel e in the stem to the diphthong ie, and the vowel o to ue.