Learn Italian: Siamo andati in piscina.

In Italian, the past tense usually uses avere (to have) as a helper verb. But for verbs about moving from one place to another, like andare (to go), you must use essere (to be). So we use siamo (we are) instead of using abbiamo (we have).

Why does andati end in an -i?

When you use essere to form the past tense, the action word (the past participle) acts a bit like an adjective. It has to match the people doing the action. Since we are talking about a plural group (we), the ending takes the masculine plural -i.

What if the group saying this is entirely female?

If it is a group of only women, the ending of the action word must match them. It changes to the feminine plural -e, making the sentence Siamo andate in piscina.

Why is it in piscina and not alla piscina for to the pool?

Certain common places in Italian always use the preposition in without an article (like the) when you are talking about going to them. Piscina is one of these memorized exceptions, just like banca (bank) and montagna (mountain).