The tense of a verb is determined by when the action took place.
Simple Present: They walk Present Perfect: They have walked Simple Past: They walked Past Perfect: They had walked Future: They will walk Future Perfect: They will have walked
The three main tenses are:
past tense present tense future tense
The past perfect tense designates action in the past just as simple past does, but the action of the past perfect is action completed in the past before another action.
The vegetables were raised before they were sold.
In (1), Yunni waited until Nick arrived and then washed the car. In (2), she had already finished washing the car by the time he arrived.
In sentences expressing condition and result, the past perfect tense is used in the part that states the condition.
The future perfect tense designates action that will have been completed at a specified time in the future.
Note: There can be only one “would have” action group in a sentence.