The reason why the 'ils/elles' form of 'venir' is 'viennent' instead of 'vienent' is because in French, you can't have a stressed /ə/ sound before a consonant and a mute e, so it has to accommodate by changing the /ə/ to an /ε/, this time by doubling the letter 'n.' 'Viens' and 'vient' don't have this change because -s and -t are both consonants, but the 'ils/elles' suffix -ent begins with a vowel, changing the 'vien-' from a closed syllable to an open syllable.
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Anglais example sentences with "Viens"