-han, -hän (particle used for emphasis)
Finnish has a number of enclitic (words that are pronounced with very little emphasis and join to the word before them, like the "'nt" in "can't") particles that are used to indicate emphasis, surprise, questions, and so on. They follow vowel harmony with the word they are attached to, that is, -han will be used after words that have any of the vowels a, o, or u, whereas -hän will be used after words that only have some of the vowels e, i, y, ä, ö.
For example:
Jussihan on tänään iloinen.
"I say, Jussi is quite happy today."
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So Jussi has both a “u” and an “i”. The suffix “han” was chosen. Why not “hän” since that suffix is used after words that have some of the vowels e, i, y, ä and ö.
I am sure the answer is crystal clear but I do not see it. Help, please and kiitos.