Subject- Verb Agreement.
A singular subject should have a singular verb and a plural subject should have a plural verb. Further, if the subject is of Ist, IInd , or, IIIrd person the verb should be the same. The usual subjects we have belong to the third person, even though first and second person subjects are also quite common. In addition to pronouns we also have singular and plural nouns as subjects and the verbs can be `regular` verbs like sit, stand, walk etc. Can we solve questions with these by substituting pronouns for the noun subjects as follows:
We know that the pronouns are I, You, He , She & It [singular] and We, You, and They [plural]. Suppose the sentence is John [live/lives]next door. Here, we can see that the subject is `John` which we can substitute with the pronoun `He`. Then, the sentence reads: He [live/lives] next door and going by the sound we can choose `lives` as the answer.
Next, suppose the sentence contains a plural subject as in : John and Tom [walk/ walks] out of the room. Here the subject is John and Tom which we can substitute with the plural pronoun `They`. So going by the sound of the sentence the answer reads: They walk out of the room. Another example is : `John and Tom[ doesn`t/ don`t ] know the answer `, which by pronoun substitution becomes `They [doesn`t/ don`t] know the answer`. Then the answer is `They don`t know the answer`. [If we don`t want to go by the sound then we have the usual rule of subject-verb agreement that says Third person singular subjects take a verb which is plural in form and Third person plural subjects take a verb which is singular in form as in:
He kicks the football & They kick the football].
But, it is necessary to have a thorough knowledge of subject - verb agreement rules because of the error of proximity which makes us choose a verb which agrees with the nearest subject and not the real subject as in: The use of credit cards [has/have] increased . Here the real subject is ` The use` and not `credit cards`. So , if we were to substitute a pronoun we should choose the singular `It` and not the plural ` They` . So, the answer to this becomes `It has increased`.
Similarly, in a sentence like `Time and Tide [wait /waits] for no-one`, we should remember that `Time and Tide` form a single pair [is this an oxymoron?]so we should substitute the singular `It` and not the plural `They` and choose the singular `waits` as the answer. So the answer is Time and Tide waits for none.