question about が and 代わりに in this sentence


> 先生がかわりにしてあげます？

I came across this sentence and I cant seem to understand why が is used here.
I'm roughly interpreting this as: "shall I substitute for the teacher?/shall I
do x in place of the teacher?" But I'm having trouble understanding why が is
used here instead of の since I feel like I've seen this same structure used
with の before and I've only seen の-が being interchangeable in relative
clauses.




This 先生 is a simple subject marker. 先生 serves as the subject of する. This
sentence may be easier for you to understand if the word order is changed a
bit:

> かわりに、先生がしてあげます？  
>  Instead, will I (=Sensei) do it for you/them?

I believe this sentence was said by the 先生. In other words, 先生 is used as a
first-person pronoun, which is very common in school.

かわりに ("instead") is an adverb that is used alone here. It may be "instead _of
you/them_ " or "in place _of you/them_ " depending on the previous context
(but not "instead of the teacher"; that would be 先生 **の** 代わりに).




This 先生 is a simple subject marker. 先生 serves as the subject of する. This
sentence may be easier for you to understand if the word order is changed a
bit:

> かわりに、先生がしてあげます？  
>  Instead, will I (=Sensei) do it for you/them?

I believe this sentence was said by the 先生. In other words, 先生 is used as a
first-person pronoun, which is very common in school.

かわりに ("instead") is an adverb that is used alone here. It may be "instead _of
you/them_ " or "in place _of you/them_ " depending on the previous context
(but not "instead of the teacher"; that would be 先生 **の** 代わりに).

