The Praxis writing section is predominantly a grammar test, and grammatical rules are a matter of memorization and recognition, not common sense. If you don't know what a semicolon is for, you have no way to reason it out once the exam starts, so you'd better know before you get in there.
The bad news is that any rule of English grammar could conceivably pop up on the test, but the good news is that the "Big Five" rules account for a majority of the questions. So whatever else you study, make sure you know these five rules inside-out:
Comma splices: You can't separate two independent clauses with just a comma. You need to use a comma and a conjunction, a semicolon, a colon, or a single dash, or you need to make them two separate sentences with a period.
Misplaced modifiers: When a sentence opens with a descriptive clause or phrase, the person or thing that it's describing needs to come right after the comma that follows it. For example, the sentence "While playing football in the house, the lamp got broken" would be incorrect because it implies that the lamp was playing football. It should be "While playing football in the house, we broke the lamp."
Subjective versus objective pronoun case: Pronouns require knowing whether to use "I" or "me," "he/she" or "him/her," "we" or "us," "they" or "them," and (the dreaded) "who" or "whom." The rule is that you use the subjective case of a pronoun (I, he/she, we, they, who) when the pronoun is performing the verb, and the objective case (me, him/her, us, them, whom) when the pronoun is the object of a verb or preposition, as in "He taught me," "Throw the ball to him," or "Whom do you love?" Watch out for the trick where the sentence places another noun in between the verb or preposition and the pronoun, as in "Please take Tom and me home now."
Tense and number agreement: Always scan your answer choices to make sure that everything makes logical sense in terms of verb tense. For example, the sentence "Before I gave him the book, he gives it to me" doesn't make sense, because something that happens in the present tense couldn't have occurred before something that happened in the past tense. There are also be a bunch of singular/plural tricks, so watch out for those — the sentence "If a student wants to do well, they have to study" is incorrect because the singular noun student doesn't match the plural pronoun they.
Parallel phrasing: In short, don't mix up the infinitive form (the kind preceded by "to") and the participial form (the kind that ends in "-ing") of a verb. Neither one is preferable all the time, but the forms shouldn't be mixed. You can say "I like to write and read" or "I like writing and reading," but you should avoid saying "I like to write and reading."