posted by Henry2 .
I just have a doubt on a few sentences.
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.
1. Yes, has
Rather than IN the porch, ON the porch