Verb Errors

A verb must agree with its subject

Sometimes an "s" is placed on the end of a verb, and sometimes it is not. 
For example, we say "Bob drives a car" but "I drive a car."An "s" is used at the end of present tense verbs when the subject of the verb is third person singular (he, she, it, Mary, the dog...). When the sentence is simple, it is fairly easy to choose the correct verb.  If you are a native speaker of English, the following sentences will likely sound incorrect to you:

Incorrect: Mary play volleyball on Thursdays. 
I loves it when you sings that song.

Obviously, the sentences should read...

Correct: Mary plays volleyball on Thursdays. 
I love it when you sing that song.

However, when sentences get more complex, it is easier to mistakenly use a verb that does not agree with its subject. There are several ways this can happen.

Several words in-between subject and verb

Incorrect: The dogs that we found roaming in the field belongs to Mr. Jones.

This mistake occurs because the subject and verb are not next to each other, as they often are.  If you ignore the modifying clauses and phrases between the subject and the verb, the correct choice becomes obvious: The dogs... belong to Mr. Jones. 

Thus, the sentences should read...

Correct: The dogs that we found roaming in the field belong to Mr. Jones.

Two subjects for one verb

Incorrect:  Julie's natural charm and ability to communicate makes her the logical candidate for the job.

Because charm and ability are two things, we must use the same verb we would use with if the subject were the word "things": These things make her the logical candidate for the job.  Thus, the sentence should read

Correct: Julie's natural charm and ability to communicate make her the logical candidate for the job. 

If the two subjects are joined by "or" or "nor," then use the verb that agrees with the subject closer to it:

Correct:Neither Julie's natural charm nor her ability to communicate makes her a logical candidate for the job.

Subject comes after verb (often in sentences starting with "Here" or "There")

Incorrect: There goes those two guys that we met at the concert.

Note that the words "here" and "there" can never be the subject of a sentence.  To find the subject, ask yourself, "What or who is performing the verb?"  In this case, "Who goes?"  The answer is "two guys."

If the subject came before the verb, the correct answer would be easier to spot: Two guys go there. The original sentence should read

Correct: There go the two guys that we met at the concert.

Avoid needlessly shifting verb tense

Sometimes, if the content of a paragraph requires it, a writer shifts between past and present tense:

Correct:  When I was in high school, I wanted to be an accountant (past tense), but now I think I want to be a teacher (present tense).

Keep your entire essay in the same tense unless the content requires you to shift.  In other words, only shift verb tense when you have to.

Incorrect (needless tense shift): Hamlet hesitated when the ghost told him to kill his uncle (past tense).  This hesitation leads to the play's tragic ending (present tense).

Pick one tense or the other and stick with it.

Correct (all in past tense): Hamlet hesitated when the ghost told him to kill his uncle. This hesitation led to the play's tragic ending. 

Correct (all in present tense): Hamlet hesitates when the ghost tells him to kill his uncle. This hesitation leads to the play's tragic ending.

Literature is usually discussed in present tense, so the second sentence would be the better choice in this case.