Learn Italian: Ieri sera mi sono rilassato sul divano.

Because rilassarsi (to relax) is a reflexive verb, which you can spot by the mi (myself) right before the verb. In Italian, all reflexive verbs use essere (to be) as their helper verb in the past tense, rather than avere (to have).

If a woman is speaking, does rilassato change?

Yes! Because this past tense uses essere (to be) as the helper verb, the ending of the past action word (the past participle) has to match the gender of the person speaking. A female speaker would say mi sono rilassata.

What two words combine to make sul?

Sul means 'on the'. It is a combined word made by joining the preposition su (on) and the masculine singular word for 'the', which is il (to match il divano).

Why is 'last night' translated as ieri sera instead of using the word for night, notte?

Ieri sera literally means 'yesterday evening'. Italians use sera for the time of day between late afternoon and when you go to sleep. Notte is used for the deep night when people are sleeping. Since you are awake and relaxing on the sofa, it is still the sera.