I just have a doubt on a few sentences.

Question

Thank you.
1) A pair of robins has (not have) been building a nest in the porch since last week.
2) If you want to visit the USA, you must (not have to) get a passport.
3) When Dorian sees the corrupted image on (in) the portrait, he decides to get rid of it but he misteriously stabs himself to death.

Answer 1

1. Yes, has
Rather than IN the porch, ON the porch

2) either

Sra