When to use [ -께서, -께서는 ] .
가: 누가 책을 읽어요? (머머니)
나: [My answer:] 어머니께서는 책을 읽으세요. [Why should it be 께서 instead of 께서는 ? Isn't it like 저는 책을 읽어요.]

가: 누가 이야기를 할 거예요? (할아버지)
나: [My answer:] 할아버지께서는 말씀하실 거예요. [Again, why is 께서는 wrong??]Figured it out just now. For those who were wondering :

As -께서 is the 경어 for '이/가' , -께서 is used because the question is asking with the particle '가' from '누가'.

;P

Question

When to use [ -께서, -께서는 ] .
가: 누가 책을 읽어요? (머머니)
나: [My answer:] 어머니께서는 책을 읽으세요. [Why should it be  께서 instead of 께서는 ? Isn't it like 저는 책을 읽어요.]

가: 누가 이야기를 할 거예요? (할아버지)
나: [My answer:] 할아버지께서는 말씀하실 거예요. [Again, why is 께서는 wrong??]Figured it out just now. For those who were wondering :

As -께서 is the 경어 for '이/가' , -께서 is used because the question is asking with the particle '가' from '누가'.

;P

Answer

~께서 comes with verb for action. Like read, talk, walk, etc...
~께서는 comes with be verb "be" Like He is a teacher, He is there, etc...
Of course there can be some exceptions, but normally...