As a copyeditor, a common grammar issue I find in a piece of writing (besides punctuation problems) is subject-verb agreement difficulties.
In simplest terms, in any sentence, verbs must agree with their nouns in number, whether singular or plural. Look for the subject of the sentence and don’t be distracted by intervening prepositional phrases, the may be the most common cause of subject-verb agreement problems. Consider the following example:
“The box of chocolates (has/have) fallen on the floor.” The correct verb is has, although many writers would mistakenly write have. How can you be sure? Trust your ear. Remove the prepositional phrase (of chocolates) and hear what makes sense: “The box [singular subject] has fallen [singular verb] on the floor.”
Another example: “The main argument of the defense attorneys (is/are) that the defendant wasn’t anywhere near the scene of the crime that day.” Remove the prepositional phrase (of the defense attorneys) and what remains? “The main argument [singular subject] is [singular verb] that the defendant wasn’t anywhere near the scene of the crime that day.”
Questions or comments? Let me know.