It is incorrect.
You may use
"You saw him yesterday" as a statement
or "You saw him yesterday?", as a question
Depending of if it is a question or a statement: Did it rain yesterday? OR It rained yesterday.
no, it's not a correct answer.
Yes.Yes.Yes.Yes.
No this is incorrect grammar. To make the sentence grammatically correct you should say: One of my cousins came yesterday.
The sentence is almost correct. However, it should be "You sent it through DHL yesterday."
"You saw him" would be correct in this context.
yes. She felt sad yesterday is correctly structured.
Yes, the sentence "Yesterday Tom washed the car" is grammatically correct.
both are correct but first one is a question and second is a sentence
I could have helped you yesterday
No, the correct form would be "I saw him twice yesterday." Using the past simple tense "saw" instead of "had seen" is more appropriate for describing completed actions in the past.
The former needs a subject, i.e. I did it yesterday. The latter requires a helping verb and a subject, i.e. I haddone it yesterday. With proper conjugation, they are both correct.