"Si sono messi i loro pantaloni."

Translation: They have put their pants on.

[val234]

So when you use a verb in a reflexive manner and in passato prossimo, you always use 'essere' as the auxillary?

[f.formica]

Yes, as a rule all reflexive verbs use 'essere' as auxiliary, regardless of the non-reflexive form.

[laura65]

I don't understand why sono was not allowed to refer to the first person in this sentence.

[f.formica]

Because of 'si' (reflexive of the third person) and 'messi' (plural); "I have put my pants on" would be "Mi sono messo i miei pantaloni". "I have put their pants on" would be odd, but it'd be "Mi sono messo i loro pantaloni".

[laura65]

Thanks, I get it now

[Eylon.saadon]

where is the "to have" verb?

[aviatorjenna]

Technically there is no "to have" in the sentence (word wise). With the passato prossimo, this sentence can say "they put on" or "they have put on." Either works.