Qual é a diferença entre I couldn't recognize my old friends because I hadn't met them for years. e I couldn't recognize my old friends because I haven't met them for years. ?Podes indicar apenas respostas exemplo.

You use the past perfect with the past tense, and you use the present perfect with the present tense. For example:

When I saw (past tense) my old friends, I didn't recognize them because I hadn't seen (past perfect) them in years.

I haven't seen (present perfect) my old friends in years, so even if I saw* them right now (present tense) I wouldn't recognize them.

*In the second sentence, "saw" looks like past tense, but it says "if I saw" - that means it is the subjunctive. "If I saw them *right now* - that means it is actually present tense.