Learn Italian: L'associazione che abbiamo visto ieri è nuova. - The association that we saw yesterday is new.

In Italian, che is the most common relative pronoun used to connect two parts of a sentence. It can mean "who", "whom", "that", or "which" and it never changes its form for gender or number. You use it when it acts as either the subject or the direct object of the verb, as it does here.

Since l'associazione is feminine, shouldn't the past tense be abbiamo vista instead of abbiamo visto?

This is a great catch! When you use a regular direct object pronoun like la (it), the past participle must agree (like l'abbiamo vista). However, when the direct object is the relative pronoun che, making the past participle agree is optional and usually skipped in modern Italian. Leaving it as the default masculine singular visto is perfectly correct and more natural here.

How do we know associazione is feminine so we know to use nuova?

A very handy rule to remember is that almost all Italian nouns ending in -zione are feminine. Because l'associazione is feminine and singular, the adjective describing it at the end of the sentence must also be feminine and singular, which is why we use nuova instead of nuovo.