I have seen that sometimes the honorific is used for only some part of the sentence, such as at the subject particle or verb. For example,

할머니가 식용유를 사려고 스퍼마겟에 가셨어요.

In this sentence, the honorific is not used at the subject particle (께서). In other cases, honorifics are not used at all.

So I wonder how to decide if I use the honorifics and on which part to use?

I have seen that sometimes the honorific is used for only some part of the sentence, such as at the subject particle or verb. For example,

할머니가 식용유를 사려고 슈퍼마켓에 가셨어요.

In this sentence, the honorific is not used at the subject particle (께서). In other cases, honorifics are not used at all.

So I wonder how to decide if I use the honorifics and on which part to use?