Son, sa, ses: they agree with the thing
CEFR A1 · part 2
edumaterial's parts within the level. The European framework does not subdivide them.
The partitive: du, de la, desUsing du, de la or des before uncountable things, and switching to de after a negative.à and de with the article: au, aux, du, desContracting à + le into au, à + les into aux, de + le into du and de + les into des…Adjectives: before or after the nounPlacing the adjective correctly and making it agree, including irregular feminine forms.Passé composé: avoir or être?Choosing the auxiliary, forming the participle and agreeing when required.C'est or il estUsing c'est to introduce or identify —with an article after— and il/elle est to describe…The three ways of asking a questionAsking the same question three ways and knowing which each situation calls for…The rule, in three lines
The French possessive agrees with the thing owned, not the owner: son livre (his or her book), sa voiture, ses amis. Before a vowel it is always son: son amie. And with several owners, leur / leurs.
The usual slip. Choosing by the owner. Sa voiture is both «his car» and «her car»: what decides is that voiture is feminine.
Why it catches you out. English chooses by the owner —his car, her car— which is the opposite rule. So you spend the whole exercise thinking about the wrong noun, and nothing in the sentence warns you.