Now read the following sentences:
* I write.
* I am writing.
* I have written.
* I have been writing.
The verbs in all of these sentences refer to the present time, and are, therefore, said to be in the present tense.
In sentence 1, however, the verb simply talks about the action. It doesn’t state whether the action is complete or not. The verb is therefore said to be in the simple present tense.
In sentence 2, the verb shows that the action is incomplete or continuous.The verb is therefore said to be in the present continuous tense.
In sentence 3, the verb shows that the action is finished, complete or perfect at the time of speaking. The verb is therefore said to be in the present perfect tense.
In sentence 4, the verb is said to be in the present perfect continuous tense because it shows that the action which started at some point of time in the past is still continuing at the moment of speaking.
Just as the present tense has four forms, the past tense also has four forms.
* I wrote. (Simple past tense)
* I was writing. (Past continuous tense)
* I had written (Past perfect tense)
* I had been writing. (Past perfect continuous tense)
Similarly, the future tense has the following four forms:
* I will/shall write. (Simple future tense)
* I will/shall be writing. (Future continuous tense)
* I will/shall have written. (Future perfect tense)
* I will/shall have been writing. (Future perfect continuous tense)