Concept of Tense in English Grammar
The second language learners face unique kind of problems while dealing with the tense category of English grammar. To grasp the concept you have to climb the ladder step by step.
First, it is important to define the term 'tense' itself. To do so, you need to properly define verb category in English grammar. The verb shows actions, states, processes and events. For instance:

They assess the report. (Process verb)
John eats an apple. (Action verb)
This is a book. (State verb)
Ali built a house (Event verb)

Now, in order to understand the concept of tense first we have to understand the concept of time. Time is a mathematical entity. It is also universal because its start and end cannot be determined. Therefore time is like a straight line on which we can show past, present and future time:
Past Present Future

That much about the concept of time. What is tense then? Tense = verb (which is state, process, action, event) + Time. Thus in the sentences below the verb notion remains intact but only the time will change by changing the form of the verb:

1. They assess (process+present time) the report. The assessed (process+ past time) the report.
2. John eats (action+ present time) an apple. John ate (action+ past time) an apple.
3. This is (state+ present time) a book. This was (state+ past time) not a book.
4. Ali builds (event+ present time) a house Ali built (event+ past time) a house

It is clear that the grammarians have created various forms of verb to capture time.
However, there are only two tenses in English grammar: past and present. There is no future tense for we have no future form of verb:

Present Past Future
eat/eats ate

Even then there are different ways to show future time:

1. He will eat an apple.
2. He is going to eat an apple.
3. He leaves for Islamabad tonight.
4. He is leaving for Dubai tomorrow.
5. He is about to leave.

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