Hi, Konstantin.
Because German
deinen is the Akkusative form for English
your, BUT, if
you are a single person,
like:
deinen Sohn.
But if
you are more than one person, than German Akkusative masculine for
your will be
euren, i.e.:
euren Sohn,
Compare the following three sentences:
1.
Hi, Tom. I have seen your son today. = Hi, Tom. Ich habe heute deinen Sohn gesehen.
2.
Good day, Mr Wolker. I have seen Your son today. = Guten Tag, Herr Wolker. Ich habe heute Ihren Sohn gesehen.
3.
Hi, Tom and Maria. I have seen your son today. = Hi. Tom und Maria. Ich habe heute euren Sohn gesehen.
So,
1. you are talking to a single person, informally, like to your friend, child,...
2. you are talking to a single person, formally, like your teacher, director, client, ...
3. you are talking to
more than one person,
And, to answer that sentence correctly you have to look what it says in the brackets:
your(pl.) - plural (euren), your(sg.) - singular (deinen), Your - formal (Ihren).
Here is the table of the German pronouns in the Akkusative case, use it as your reference at:
German Pronouns: Accusative case
Also see the conjugation of the German verbs with examples on this site at:
German Verbs reference with examples
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