She is not a person who you can count on.
or
She is not a person whom you can count on.
Which one is correct and sounds more native ? Please give an explanation for your answer
It is "whom" when used as the object of the sentence, and "who" when used as the subject.
Since the person you are counting on is the object of the sentence, "whom" is technically correct.
However, very few native speakers would say "whom" in normal speech, since few native English speakers understand when to use it.
"Who/whom" is a relative pronoun- it "relates" the second clause to the first clause, and acts as either subject (who) or object (whom) in the second clause. Look at these two sentences, and see what the second clause looks like on its own:
She is a person who gets things done
she gets things done - subject
She is not a person whom you can count on
You cannot count on her - object
English speakers usually avoid this issue altogether. There are two ways that you can do this:
She is not a person that you can count on
that is also a relative pronoun, but it does not change depending on whether it's a subject or object
She is not a person you can count on
You can often omit the relative pronoun: in this case it is allowed. You will find the rules here.
In this case, "who/whom" is the object of the preposition "on", so you should use the objective case of whom. If you were speaking very formally, you might say
She is not a person on whom you can count.
That is probably how you would hear it on Downton Abbey, and there are people that tell you never to end a sentence with a preposition, but it happens all the time in spoken language. In American English, I think you are more likely to hear it said as you had it in your first example (technically incorrect, but much more common)
She is not a person who you can count on.
Or, as @JavaLatte suggested, it would probably just be omitted entirely.
She is not a person you can count on.
Think of it this way: you would say You can count on her
, not You can count on she
. That of course is because on
is a preposition which is followed by its object, her
. In your sentence, who/whom
is still an object of on
and therefore should take that form: whom
. Actually, to be technically correct, that sentence should be written as She is not a person on whom you can count.
Whenever you are wondering if you should use who
or whom
, you can usually say, Would I use he or him?
Of course he
translates to who
and him
translates to whom
. Thus, I will follow him.
would become I will follow whom?
or Whom will I follow?