Learn Italian: Lui ha aspettato la pensione per dieci anni. - He waited for retirement for ten years.

We use per because the waiting is a completely finished action. When you talk about a duration of time that has ended, you use per. If we used da (since/for), it would mean he started waiting ten years ago and is still waiting today.

Why isn't there a word for 'for' after ha aspettato?

In English, we say to wait 'for' something, but the Italian verb aspettare takes a direct object. You don't need a preposition; you just directly connect the verb to the thing being waited for (ha aspettato la pensione).

Why does the sentence use the article in la pensione when the English translation just says 'retirement'?

Italian uses definite articles like la for abstract concepts and broad categories, unlike English. So, retirement becomes la pensione.

Because he waited in the past, why don't we use the imperfetto tense (aspettava)?

The passato prossimo (ha aspettato) is used here because the action has a clear, defined duration that is totally finished (per dieci anni). If you used the imperfetto, it would sound like an ongoing background habit without a clear end.