Hello. Please, can you help me? I never dont know when I use in sentence- the, a or an. It is terribly confusing me, and I must to know that of course. It brings me various problems in correspondence and can someone help me?

Except that I am not sured nor with Past tense, of Perfect Tense, when I use on, and when the other? Perfect Tense - we use when we dont know when that activity happenned, but I see that sometimes we use for that and Past tense. For example
I done my dinner or I have done my dinner? thank you¨!