Learn Italian: L'amicizia è importante quanto l'amore. - Friendship is as important as love.

In Italian, the full way to say 'as [adjective] as' is tanto [adjective] quanto. However, it is very common to drop the tanto. So, è importante quanto l'amore is simply the everyday, shorter version of è tanto importante quanto l'amore.

Why are there definite articles before amicizia and amore? We don't say 'the friendship' in English.

Italian requires definite articles (il, la, l', etc.) for abstract concepts and nouns used in a broad, general sense. While English drops the article for broad ideas like 'friendship' or 'love', Italian always keeps it.

Since this lesson is about the imperfect subjunctive, how would I say 'I thought friendship was as important as love'?

You would say: 'Pensavo che l'amicizia fosse importante quanto l'amore.' Verbs of opinion in the past, like pensavo che (I thought that), require the imperfect subjunctive. Therefore, the present indicative è changes to the imperfect subjunctive fosse.

Does quanto ever change its ending to match the nouns it compares?