Hi, if I'm not mistaken, "seen" and "done" are
past participles, "have seen" and "have done" are verbs in the present perfect tense, and in those verbs "have" is an
auxiliary verb.
Also, "saw" and "ate" are verbs in the simple past tense.
When you learn a new verb, you need three forms of that verb, infinitive, past tense, and past participle:
eat (bare infinitive) ------------ ate (past tense) ------------ eaten (past participle)
see (bare infinitive) ------------ saw (past tense) ----------- seen (past participle)
... and so on.
Past participles are used with auxiliary verbs (but sometimes they can also be used as adjectives):
I have eaten that stuff --------> (present perfect tense) auxiliary "to have" + past participle
That stuff was eaten ----------> (passive form) auxiliary "to be" + past participle
That stuff has been eaten ---> (passive form) auxiliary "to be" +
past participle (NB: the auxiliary verb is "to be" in passive forms. In
this example, it's in the present perfect tense, has been, which needs
"to have" as auxiliary verb. That's why in the end we get has + been +
eaten, one auxiliary verb and two past participles)
I don't know if that can help you at all