A possessive adjective indicates ownership of something and in Italian they would translate as my, hers, yours, theirs, ours... There are different forms depending on who we are talking about- for instance, if I was talking about myself I would use of of the forms of mio- the exact form depends on the gender and the number of the noun being possessed. If I was talking about them over there, the 'they' category grammatically speaking, I would use one of the forms of 'loro''.
So, the Italian forms are as follows:
Singular: il mio, il tuo, il suo, il nostro, il vostro, il loro.
Plural: i miei, i tuoi, i suoi, i nostri, i vostri, i loro.
Singular: la mia, la tua, la sua, la nostra, la vostra, la loro.
Plural: le mie, le tue, le sue, le nostre, le vostre, le loro.
So, now we have seen the forms themselves, we'll have a look at some of their rules.
One of the first things to note is that they agree in gender and number with the noun possessed, not with the possessor. So, for example, a boy called Bob wants to talk about his car. But, in Italian, car = la macchina (a feminine noun). So, we would use the feminine singular form of 'sua'- la sua, regardless of the fact that Bob is male. Thereofre = la sua macchina. His car. The possessive adjective agrees with the noun being possessed. We'll take another example: Bob wants to talk about his two pizzas. Now, pizzas are feminine, and also, we need to take into consideration that Bob has two of them (remember as we have seen above, there are different versions of the adjectives for singular and plural) So, regardless of the fact that Bob is male and that there is only one of him, we would use the feminine plural form: le sue. Therefore = le sue pizze. His pizzas
Something else to note: Loro is invariable in gender and in number. When other forms like mio change (mio, mia, miei, mie) loro always stays the same.
Also, as we have seen, unlike in English the Italian possessive adjectives are also preceded by definite articles (il, la, i, le), so if we were to translate them directly back into English, it would be like saying: 'the my book', 'the her car'.
This is practically always the case and the definite article is virtually always present, but, well ,grammar wouldn't be so exciting if there wasn't always some lovely exceptions! So, here goes:
1) Never use the article with close family members
So we would say, mia madre, mio padre, sua zia, suo nonno..
2) But, DO USE THE ARTICLE with plural family members.
So, we would say, le mie zie but mia zia.
3) USE THE ARTICLE with definite family members- those with an adjective- so la nonna materna, il zio di napoli...
4) COLLOQUIAL TERMS FOR FAMILY MEMBERS NEED AN ARTICLE: la mia mamma, il vostro papa.
5) Altered nouns relating to family members do need an article- these are nouns with suffixes (-etto, ello)
Eg: il mio cuginetto. My little cousin.
6) And just to make things confusing, non of these rules apply to 'loro'- their. (as we saw earlier, it's a bit of an odd one out) In the singular and in the plural, a definite article is always needed.
Now, with all this to remember, it is perhaps pleasing that these possessive adjectives are not in fact used as extensively in Italian as they are in English.
1) They are not used when it is clear who something belongs to. Eg: ho lasciato a casa il passaporto- it's clear that I left my passport at home, so we don't need to say it.
2) They are not used to refer to body parts/ clothes/ hair/ personal belongings. So in Italian, you can say: Voglio lavarmi i cappelli oggi - we don't need to say 'my' hair. Mi fa male il braccio- my arm hurts. We know from the sentence that we are discussing 'my' arm.
And to finish up, a few odd rules:
1) The possessive adjective usually always precedes the noun, but sometimes it doesn't. As in the following cases:
a casa mia (my house)
è colpa sua (it's his/her fault)
è merito tuo (it's your merit)
piacere mio (my pleasure)
Never say vado alla mia casa- this is wrong. say Vado a casa mia.
2) i miei, i tuoi, i suoi depending on the context, can be short for i genitori (parents). So we can say, i miei hanno un gato. My (parents) have a cat.
So, I hope this helps. In terms of learning them, I would suggest if you wrote them out in collumns- all the masculine ones - having the singular on the left and the plural on the right, then the feminine.. And if you are stuck thinking about their endings, just think back to the normal adjective endings: -o is masculine singular, -a is feminine singular, -i is masculine plural, -e is feminine plural.