PRESENT PERFECT
Formula :
The English Present Perfect tense is used to express actions which have already been completed, or perfected, at the time of speaking or writing. In the examples given below, the verbs in the Present Perfect tense are underlined.
Example :
I have done the work.
She has answered half the questions.
In the first example, the use of the Present Perfect tense emphasizes the fact that, at the time of speaking or writing, the work has already been completed. In the second example, the use of the Present Perfect indicates that, at the time of speaking or writing, half the questions have been answered.
PRESENT PERFECT CONTINUOUS
Formula :
The Present Perfect Continuous tense is used to express continuous, ongoing actions which have already been completed at the time of speaking or writing.
In the following example, the verb in the Present Perfect Continuous tense is underlined.
Example :
The bus has been waiting for one hour.
The use of the Present Perfect Continuous tense in this example indicates that, at the time of speaking or writing, the bus has completed one hour of continuous waiting.
PAST PERFECT
Formula :
The Past Perfect tense is used to refer to a non-continuous action in the past, which was already completed by the time another action in the past took place. In the following examples, the verbs in the Past Perfect tense are underlined.
Example :
She had heard the news before I saw her.
I had finished my work by the time the clock struck twelve.
In the preceding examples, the verbs had heard and had finished are in the Past Perfect tense, and the verbs saw and struck are in the Simple Past. The use of the Past Perfect tense indicates that the actions of hearing the news and finishing the work were already completed by the time the actions expressed by the verbs in the Simple Past took place.
PAST PERFECT CONTINUOUS
Formula :
The Past Perfect Continuous tense is used to refer to a continuous, ongoing action in the past which was already completed by the time another action in the past took place. In the following examples, the verb in the Past Perfect Continuous tense are underlined.
Example :
I had been waiting for two months by the time I received the reply.
He had been thinking about his friends shortly before the called.
In the preceding examples, the verbs had been waiting and had been thinking are in the Past Perfect Continuous tense, and the verbs received and called are in the Simple Past. The use of the Past Perfect Continuous tense indicates that the actions of waiting and thinking were continuous, and were completed by the time the actions expressed by the verbs in the Simple Past took place.
SUBJECT - VERB AGREEMENT
Formula :
Just as pronouns must agree with their antecedents in person, case, and number, verbs also must agree with their subjects in person and in number.
The first person subject is the person or persons speaking in a sentence (I, we). The second person subject is the person or persons addressed (you, you). The third person subject refers to the person or thing spoken about and may be any noun or third-person (he, she, it, they).
Verbs agree with their subjects in number. Therefore, a singular subject takes a singular verb, a plural subject takes a plural verb.
Example :
The window is open.
Many computers are innovated from day to day.
Some companies need a manager who has multiple skills.
A child cries for ice-cream.
NOTE: Verbs have an opposite rule if we compare to nouns. Nouns always change their form when they are plural and keep the same form when they are singular. But for verbs, they always change their form when they are singular and keep the same form when they are plural.
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SOURCE :
Marjorie Fuchs, Bonner Margaret, Grammar Express for self study and classroom use, Addison Wesley Longman 2OO1.
Mary Ansell, Free English Grammar.
