あなたは俺が守る

Toritoribe said:

Because the verb 守る doesn't take が as the object marker.

あなたは俺が好きだ/憎い/必要だ。
the subject: あなた, the object: 俺

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nhk9 said:

The nuance difference can be understood as follows:
俺は貴方を守る I will protect you/ I protect you (as a habit/duty)
俺が貴方を守る I am the one who will protect you (and not another person who is going to protect you)
俺が貴方は守る As for you, I am the one who will protect you (and I don't know if I am going to protect other people). Notice that this type of sentence is often said as 貴方は俺が守る
When you study the language more, you will see that the particles can overlap in their functions.

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