1) Here are some links with better explanations than I could probably give.
http://www.englishpage.com/verbpage/presentperfect.html
http://www.englishpage.com/verbpage/pastperfect.html
2) "An" is easy. You always use it before you talk about a noun that starts with a vowel. Exception: if the noun starts with a silent h
"Is than an elephant over there?"
"I was in an accident last week."
"I'll see you in an hour."
"A" is used the same way as "an" except you use it before a noun or an adjective that starts with a consonent. Exception: if the noun or adjective starts with a "u" that sounds like "y"
"Is that a pink elephant over there?"
"I was in a wreck last week."
"That is a unique dress that you are wearing."
The difference between "the" and "a/an" maybe be a little confusing. You generally use "the" when you're talking about something specific.
"Please pass the milk." (As in, there is only one container of milk on the table.)
"Please pour me a glass of milk." (You just want any glass of milk, not a specific one)
"I need to go to the store to buy milk" (I don't know how to explain why no article is needed here.)