SAT Reading Question.

So, my SAT is in November and I have three main questions that I still don’t get. LOL. Anyways.

When is it grammatically correct to use has, had, or none at all? For example, “She had read” seems right, but isn’t “she read” better? I DONT KNOW. So, yeah, I need help on that.

When do you use I, or me? I’ve seen stuff like Molly and I while others, me. So, not sure about that.

What is a gerund? lol, and what are its rules?

And if you guys have any really important key points/advice for the SAT, please say so! Thanks.

This is more of a “tense” question, and depends on when you are saying the sentence (utterance time), when the actions of reading the book are done (situation time), and the time for which you are restricting the subject’s actions to (topic time). See http://www.glottopedia.org/index.php/Tense

“I” is used if it represents the subject of a sentence or clause, and “me” is used if it represents the object.

I went to the store.
Molly and me went to the store. ( denotes a grammatically incorrect statement)
He gave the book to Molly and me.
*He gave the book to Molly and I.

See https://en.wikipedia.org/wiki/Gerund