After reading all comments, this is the most helpful for understanding how this sentence works.
So basically the subject is not Bebek, but süt, and bebek için is a single part of the sentence, where için shows the relation between bebek and süt.
Get some lingots, and Çok teşekkürler!
You are right for the English sentence if you say:
- The baby needs milk.
The construction of the Turkish sentence is different so 'milk' is the subject there. As Ilkei wrote above you could literally translate:
- Milk is necessary for the baby.
In this sentence 'milk' is the subject.
I guess I get your question now! :)
You do not change nouns used with 'için'. So here 'bebek' remains as it is. Though in 'nominative form' it is not the subject of the sentence.
If you have personal pronouns you take the genitive form, e.g.:
benim için, senin için, onun için, bizim için, sizin için
(exception: onlar için)
From the way this sentence is built I understand that gerekiyor means in fact 'is needed' and the sentence literally would translate into: "for the baby milk is needed" (or: milk is needed for the baby). From the English sentence I understand that this is how to say 'the baby needs milk'. But in English the baby is the active subject in the sentence and in Turkish the milk is the active subject. Please correct me if I am wrong.