Italian: use imperfect tense or past tense?

It depends what you want to say: "Loro erano amici da molto tempo" means "They had been friends for a long time". If you want to say "They have been friends for a long time" you need to use the present tense: "Loro sono amici da molto tempo". The reason for this is harder to explain. The *real* mystery is why English uses the perfect/past tense in sentences like this when (a) logic and (b) practically every language in the world uses the present tense in sentences such as this.