FUTURE PERFECT TENSE
Future perfect tense is used to denote an action to be completed within a fixed time of future.
Eg; They will have finished their work within 5.P.M.
Here, what the example says is that, the action ‘finish’ is to be completed within 5 P.M. As 5 P.M. is at present a time in future and the action ‘finish‘ is to be completed within that time, we can say that the ‘action’ is in Future perfect tense.
We know that it is included in perfect tenses. So the forms of ‘Have’ should come as auxiliary verbs and the past participle form of the verb (V3) should come after that. But, here, as ‘will’ and ‘shall’ is used as auxiliary verbs to show that the completed action happened in future tense, and also, only the root form of the verb is used after ‘will’ and ‘shall’ as mentioned in the simple future tense , we can use only the root form ‘Have’ after ‘will’ and ‘shall’.
By keeping these facts in our mind and analysing the above example, let’s make the pattern of ‘Future perfect tense’ as,
S + will/shall + have + V3 + O
Here also, ‘will’ or ‘shall’ is used on the basis of ‘persons’ of the subject and the conditions in future tense, that is, ‘normal future’ and ‘serious future’, as mentioned in simple future tense
eg; I shall have come there by 2hrs. (As ‘shall’ is used with first person ‘I’ , it is a normal future)
S( I ) + shall + have + come ( ‘V3’ form of the verb ‘come’)
They shall have completed the construction within next year..
S ( They) + shall + have + completed ( ‘V3′ form of the verb ‘complete’)
(As ‘shall’ is used with third person ‘They’, it is the future showing ‘determination’, ‘obligation’, ‘surety’, ‘strong offer’, ‘truth’ etc….)