Let's consider this example:
A killed B -> here the subject(the doer of the action) is different from the object (receiver of the action)
A killed A -> here the subject (the doer of the action) is the same as the object (receiver of the action)
in this case we say: A killed himself. we call himself a reflexive pronoun. reflexive pronouns as I mentioned are used when the subject (doer) and the object (receiver) of an action are the same.
in your example,
You should use this book to help you with the grammar that you find difficult.
consider the infinite phrase "to help you with...", the implied subject of this phrase would be "the book", which is the subject(doer) of "help" (action), finally the receiver of the action would be "you". as you can see, the doer and the receiver are NOT the same person/thing, so a reflexive pronoun cannot be used. thus using "yourself" in the first sentence is definitely ungrammatical.
the second sentence is different though
You should use this book so that you can help yourself with the grammar that you find difficult.
here the subject of the bold part is "you", the verb (action) would be "help" and the receiver of the action would be, well, "you" since you are helping yourself! so the receiver and the doer are the same, thus it IS possible to use a reflexive pronoun.
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