Grammar is the way in which sentences are structured. When proofreading, ensure that every sentence has an object, a subject and a verb to make it grammatically correct. For example, in the sentence "The teacher spoke with the students," "teacher" is the subject, "spoke" is the verb and "student" is the object. Also, sentences should have an independent clause --- a section of a sentence that would make sense on its own were it not part of the larger sentence. For example: "The girl wept continuously, which is why her parents were worried." "The girl wept continuously" is an independent clause, while "which is why her parents were worried" is a dependent fragment clause.
Content is the manner in which ideas are presented. Content is what the writer is saying in the piece of writing. When proofreading, it is essential to find out that the ideas put across are factual unless the piece is fictional writing. Also observe the flow of ideas from paragraph to paragraph and whether this flow is coherent or makes sense.
Punctuation is how symbols and marks are utilized in sentences and the structure of the piece of writing. When proofreading, check that there is a full stop at the end of every sentence. Sentences with two main clauses should be separated by a comma and a conjunction such as "yet," "but" and "and." Sentences that are too long and winding should be broken down by a full stop into independent clauses.
Reviewing spelling involves checking that words are spelled correctly. This can be done by computer spell checkers, but they may not fully detect all spelling mistakes. Proofreading for spelling also entails ensuring that the correct word is used in a sentence. For example, a computer spelling checker will not detect as a spelling error the wrong usage of "their," "they're" and "there." It is preferable to use a dictionary to establish the spelling of a word.