With some verbs, it helps to think in Reflexive Italian and not in English
There are particular Italian verbs that don't translate directly to English.
For instance in English, we say "to have fun". In this phrase, "(to) have" is the verb and "fun" is an object, but the entire phrase is an action.
In Italian, there is a verb for this phrase and it uses the reflexive form.
The verb is called "DIVERTIRSI".
The final 4 letters tell you everything that you need to know about it-- if it ends in "si" then we know it will be reflexive--and for conjugation purposes the vowel and the letter "R" in front of "si" (in this case the vowel is "I") tell us that you conjugate normally as an "IRE" verb and use that with the reflexive pronoun.
Do you remember the reflexive pronouns? Those are as follows:
- mi (myself)
- ti (yourself)
- si (himself/ herself/ itself/ oneself)
- ci (ourselves)
- vi (yourselves)
- si (themselves)
So then; you drop the "IRSI" to get the root infinitive of the verb ("DIVERT"), conjugate as you would with a normal IRE verb, and then add the appropriate reflexive pronoun:
- mi diverto
- ti diverti
- si diverte
- ci divertiamo
- vi divertite
- si divertono
Now then-- while Divertirsi "translates" in English as "to have fun", it will be much better if you think like an Italian, in this case, and translate it as "to enjoy oneself".
Equating it with "oneself" will automatically put you in mind of the reflexive, and the sentences will still "make sense" in English:
Si diverte = He enjoys himself or She enjoys herself (He or She has fun).
This also works in the passato prossimo (English simple past tense) and, indeed, all other usable tenses.
With all reflexive verbs, the auxillary verb will be "essere", and you still use the rules for IRE verbs in the passato prossimo (in the case of "divertirsi"), which would be to make it end in "ITO" for a masculine subject, or "ITA" for a feminine one. Likewise plurals; ci, vi, and si (themselves) all expect a plural ending, which would be "ITI" for masculine or mixed gender, and "ITE" for all feminine plural.
- mi sono divertito or sono divertita
- ti sei divertito or sei divertita
- si è divertito or è divertita
- ci siamo divertiti or siamo divertite
- vi siete divertiti or siete divertite
- si sono divertiti or sono divertite
Thus, in the above: Ci siamo divertiti = "We enjoyed ourselves." (a mixed group, or all males)
There are obviouslly a lot of reflexive verbs; far more than Duolingo will expose you to, and there will be some that end with ARSI or ERSI instead of IRSI, as my examples above all show.
Not to worry-- you know what to do with those now, don't you?