"You would say", suggests that the speaker thinks the listener is likely to say something like in that situation. You would say that you like my hair if I had it cut well You should say, suggests that it is required by social convention (or strict rule) that you say so-and-so. You should say thank you after you receive a gift You could say, suggest that the speaker is making a suggestion as to what the listener might say. You could say how sorry you are and you hope they get well soon when you hear they have been well.

You would say “their” instead of “there” in that context. You should say “I love you” back to him. You could say “everything” instead of “anything” in that sentence.