This shows an action that is happening or continuing at present. 

This denotes an action that was in progress at a definite time in the past.

This shows an action that will be in progress at a specific time in the future. 

 A verb tense with two main uses.

It expresses an action that will take place in some future time. The helping or auxiliary verb will or shall is used with the verb to express futurity. 

Verb tense used to describe continuing actions—actions that are in progress and ongoing.

A category of verb tense used to describe completed actions.

It expresses an action that is completed within a period of time,an action in the past that still continues to the present time, or an action done for a number of times.

It expresses a future action or condition that will occur before another future action or condition.

It expresses an action that will be in progress at a particular time in the future.