NUMBER OF VERB
In long, complicated sentences, people often lose track of whether the subject is singular or plural and use the wrong sort of verb. “The ultimate effect of all of these phone calls to the detectives were to make them suspicious of the callers” is an error because “effect,” which is singular, is the subject. If you are uncertain about whether to go with singular or plural condense the sentence down to its skeleton: “The effect . . . was to make them suspicious.”
Another situation that creates confusion is the use of interjections like “along with,” “as well as,” and “together with,” where they are often treated improperly as if they meant simply “and.” “Aunt Hilda, as well as her pet dachshund, is coming to the party” (not “are coming”).
A compound subject requires a plural verb even if the words which make it up are themselves singular in form: “widespread mold and mildew damage [not damages] the resale value of your house.”
List of errors