who ,whom :how we can difere the using between them

Question

Answer

- He = who 
- Him = whom 

Who / Whom cares for the children? 
= He cares for the children. Therefore, 'who' is correct. 
= 'Who' is an interrogative pronoun and is used in place of the subject of a question. 
It can also be used in statements, in place of the subject of a clause. 
'John is the man who got the job.' (He got the job) 

For who / whom should I write? 
I should write for him. Therefore 'whom' is correct. 
= 'Whom' is  an interrogative pronoun, but it is used in place of the object of a question. 
'With whom are you going?' 
It can be used in statements, in place of the object of a clause. 
'This is the man whom I told you about.'

Answer

For answering questions in an exam, follow Learner's advice.

For daily use/speaking, we just use "who".  We only say "whom" to sound pedantic. ;)