What kind of possessive pronouns are used after the preposition "mit"?

Question

Das Mädchen isst mit ihrem Großvater.
Der Junge schwimmt mit seinen Freundinnen.

In these sentences what kind of poessessive pronouns are ihrem and seinen?

What about this case:

Der Junge besucht seinen Großvater.

Answer 1

When dealing with possessives in German, you need to look for three things: the gender and number of the possessor; the gender and number of the possessed; and the case of the possessed. Let’s start with your second sample sentence.

• Der Junge schwimmt mit seinen Freundinnen.

Since ‚Der Junge‘ is m sg, we need to use a form of ‚sein‘ (= his). More particularly, we need to use a f pl form, since ‚Freundinnen‘ is f pl. Finally, the preposition ‚mit‘ requires Dat. Thus, we need the Dat pl f of ‚sein‘ and that’s ‚seinen‘.

• Der junge besucht seinen Großvater.

As before, ‚Der Junge‘ is m sg and we need to use a form of ‚sein‘. Since ‚Großvater‘ is m sg, we need to use a m sg form. Now, ‚besitzen‘ requires Acc: Wen besucht er? Den Großvater. Thus, we need the Acc sg m of ‚sein‘ and that’s ‚seinen‘. That’s a bit confusing since ‚seinen‘ is also the Dat f pl of ‚sein‘, as we saw. Still, it’s correct.

• Das Mädchen isst mit ihrem Großvater.

This is where things get tricky: ‚Mächden‘ is technically neuter, and not feminine! So, strictly speaking, we’d need to use the possessive ‚sein‘ again (which is both m and n). However, since Mädchen are female, it’s now common to treat the noun as though it was f. Hence, the possessive ‚ihr‘ is used. The rest is by now familiar: ‚Großvater‘ is m sg and ‚mit‘ requires Dat. Thus, we need the Dat sg m of ‚ihr‘, which is ‚ihrem‘.