I still don't get it on when should I use past tense and present tense after the word *never*.
(1) She never try/tried it before.
(2) He never tell/told me anything before he left.
(3) How could you know it's dangerous if you never try/tried it before..
Mind you I'm not a teacher but here are some hints:
You need to understand the context of a given sentence and know some rules.
You use Present Simple when something you do is a habit, a repeatable routine action. It doesn't make sense to use it in the examples you've given as they refer to the past (2) or general experience (1, 3). When you're talking about general experience you do use a present tense, but it's not Present Simple. It's Present Perfect. So:
(1) She
has never
tried it before. (In American English you could probably use Past Simple too: She never tried it before.')
(3) How could you know it's dangerous if you
have never
tried it before. - In his experience, which stretches from the past till now, he has not tried it. (Again, in American English you could probably use Past Simple too: '. . . if you never tried it before.')
Sentence (3) clearly refers to the past, the action finished in the past - 'he left' and that's that. It's not a habit or a routine of his (so we can't use Present Simple), it's not about general experience either (so we cannot use Present Perfect). What we CAN use is Past Simple:
(2) He never
told me anything before he left.
You could use Present Simple but with a different meaning:
He never
tells me anything. (That's his way of being, his routine, that's what always happens.)