confusion with future tense
I always confuse with future tenses. May I please have your description on the below:

Which of them are written grammatically correct and what is the rule?

a) I will let you know once I have finished my project
b) I will let you know when I finished my project
c) I will let you know when I finish my project

And what is the easiest way to say that I will something do after finish some work (future after future)? I understand that exist some rules like Future Perfect (ex: I will have written the letter before he comes) but I can not apply this rule here.

Question

confusion with future tense
I always confuse with future tenses. May I please have your description on the below:

Which of them are written grammatically correct and what is the rule?

a) I will let you know once I have finished my project
b) I will let you know when I finished my project
c) I will let you know when I finish my project

And what is the easiest way to say that I will something do after finish some work (future after future)? I understand that exist some rules like Future Perfect (ex: I will have written the letter before he comes) but I can not apply this rule here.

Answer

The confusion lies in the tense of the "when". For giving an action as "when" with a future form, you use present tense.

So a) and c) are correct, but b) makes no sense because the "when" tells me you finished your project in the past, so why are you making a future promise? :)

For the second question, simply use "when (or after) I finish my project, I will let you know". Again, the main action of the sentence is letting your friend know, and finishing it can be in present tense to represent "when".

Alternately, use "I'll finish my project, then (I'll) let you know."

Answer

The confusion lies with the present perfect tense. It is best to say, a) I will let you know once I have finished my project. In this case, the action completed at an indefinite time. I know that we are talking about the future, but the tense marks an "anterior aspect".

Answer

How about this sentence.  I will write the letter before he will comes or I should have written the letter before he will comes.  Using a past tense and future tense in the sentence.

Answer

The confusion is, you think it's a future tense.There is no future tense in English. "Will" is a modal, and indicates a deferred decision.

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