Each of these words has a partner: I → me, he → him, she → her, we → us, they → them. (you and it stay the same.)
The partner word
I love him.
She knows me.
They help us.
Not "She knows I." — say "She knows me.". The one who gets the action uses the partner word: I → me.
Try it: She knows ___. (me / I / my) Answer: me — If you picked "I", that is the word for the one who does it. Here she does the knowing and you get it, so use the partner word: She knows me.
After the action word
Right after an action word like see, help, call, use the partner word — that is the one who gets the action.
Can you see her?
We help them.
Call me tomorrow.
Not "I like they." — say "I like them.". After the action word, the one who gets it uses the partner word: they → them.
Try it: I like ___. (them / they / their) Answer: them — If you picked "they", that is the word for the ones who do something. Here you do the liking and they get it, so use the partner word: I like them.
After for, with, to
After the small words for, with, to, at, about, always use the partner word — never I, he, or she.
This is for you.
Come with me.
Talk to him.
Not "This is for he." — say "This is for him.". After for, switch he → him.
Try it: This is for ___. (him / he / his) Answer: him — If you picked "he", that word cannot follow for. After for, with, to, at and about, always use the partner word: This is for him.
Try it: Who gets the action? Can you help ___? (me / I / mine) Answer: me — You get the help, so use the partner word: me. ("I" is for the one who does it; "mine" says something belongs to you.)