用win or won?

I win 同　I won 是tense的分別
本身句義相同

I win present tense ( a fact that is still true)
I won past tense ( a fact that is no longer true/an event happened previously)

例句:
Today,I win the snapshot-taking competition .
I won the snapshot-taking competition held on last firday.

If no time of indication, both tenses can be used

I win/won the snapshot-taking competition.

won is past tense win is simple so u use win

I won!

I win

You would normally say "I've won!" Alternatively you could say "I won!"
Either is correct. Depending on what situation it is, you could also say "I win!"
so really, both is correct!

you mean past tense or present tense??

if you mean past tense: i won the prize!
if you mean present tense: i win the prize!

both of them are correct~