Learn Italian: Non mi è piaciuta la compagnia al matrimonio. - I didn't like the company at the wedding.

In Italian, the thing being liked is actually the grammatical subject of the sentence. Here, the subject is la compagnia (the company), which is a feminine singular noun. Because the verb piacere uses essere (to be) as its helper verb in the past tense, the past participle must agree with the subject. So, piaciuto changes to piaciuta to match la compagnia.

Why does la compagnia come after the verb è piaciuta if it is the subject?

With the verb piacere, it is very common to put the subject (the thing you like) after the verb. You could say La compagnia non mi è piaciuta, and it would be perfectly correct, but putting the thing you like at the end of the sentence is the standard and most natural-sounding word order in Italian.

Where does the non go when making the past tense of piacere negative?

The word non always goes right before the pronoun (in this case, mi). It is always Non mi è piaciuta (literally: 'not to me is it pleasing'), and never Mi non è piaciuta.

Why is it al matrimonio instead of just a matrimonio?