Hi Archi,
1 & 2: Remember that when you have a form of the verb "to do" as you do wiht "Where did I..." then you use the bare infinitive of the verb, so "to leave" is just "leave."
Do you want to leave? Where did she leave off? etc.
3. It says is present tense, It said is past tense. No, you never use "saids."
4. I would have - this requires the participle form: come. I am coming home right now, I come here often, Yesterday I came home late, I have come here many times before, I would have come to the party if I had been invited.
Yes, you can have a preposition at the end of a sentence.
Which bin did you look in? I don't know where I fit in.