Don't mistake a word in an intervening phrase or clause for the subject of a sentence. The simple subject is never in a prepositional phrase. Make sure the verb agrees with the actual subject and not with the object of the preposition.
Example: The 'paper' in those boxes 'is' for the copy machine. [The singular verb 'is' agrees with the singular subject 'paper', not with the plural object of the preposition, 'boxes'] Example: The 'dogs' in that class are well behaved. [The plural verb 'are' agrees with the plural subject 'dogs', not with the object of the preposition, 'class'.]
If a singular subject is linked to another noun by a phrase, the subject is still considered singular. Expressions such as 'accompanied by, as well as' in addition to, plus, and together with' introduce phrases that modify the subject without changing its number. Example: 'Fried rice', along with wonton soup, 'makes' a delicious meal Example: 'Isaac', accompanied by Jerome, goes to the movies on Saturday.
Appositives and adjective clauses give information about the subject but don't change its number. Make sure you don't mistake a word in an appositive or an adjective clause for the subject of the sentence. APPOSITIVES Example: 'Emma', one of my good friends, visits Australia every year. ADJECTIVE CLAUSES: 'Virginia Woolf', who was one of the Bloomsbury Group members, 'expresses' emotion in her writing.