The usage of future perfect tense

[Duc]

hello,
Please look at the two sentences below:
1.He (be) here before you go.
2.The train (leave) before we reach the station.
The two sentence are in my exercise book.The first one is in the section ''Time clauses'' and the second one is in the section ''The future perfect tense''
And the answer to the first one is ''will be'' but the answer to the second one is ''will have left'' And I don't understand their difference in tense be cause I think they should all be ''will have V-ed'',it's the usage of the future perfect tense.
Please explain it for me .
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[Molly]

He will be here before you go = he will be here when you are here ("will be here" expresses an incomplete action in the future)
the train will have left before we reach the station = the train will not be there when we are there ("the train will have left" expresses an action completed by a certain point in the future)

[Duc]

So what's about ''He will wash up before he goes to bed'' ?
"will wash up'' expresses a completed action.He finishes washing then he goes to bed.
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[Molly]

You are looking to a time when he will be washing up. The first action is the main focus.

[Duc]

but how to know what action are we focusing on because i find it difficult to see the main action in ''the train will have left before we reach the station'' ?
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[Alan]

Hi Duc,

I understand your concern about the test questions you have quoted. One of the problems when setting tests (and having written several hundred for our site I know!) is to strike a balance between suggesting a correct answer with the right amount of information and at the same time not giving too much information to make the answer obvious. In your example it would perhaps have been better if there had been some additional information such as -If we don't hurry up,

[Duc]

Thank you Alan,
And there is one thing I want to consult you about. Please read this ''In a fortnight's time,we will have taken our exam.'' What is the use of ''future perfect tense'' here? Because I don't find it in any of my grammar book.
Thanks in advance!
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[Alan]

Hi,

The meaning here as usual with the future perfect is the picture you have of a future action that is completed.

[Molly]

Here's a helpful page, duc:

edufind.com/English/Grammar/Tenses13.cfm

[Duc]

Thank you !
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[Duc]

[Yankee]

[Duc]

So Amy ,can you help me solve my problem?
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Don't see your shade think you are great

[Duc]

Any more help please?
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[Yankee]

Hi Duc

Basically, your exercise book doesn't give you the context that is necessary to help you understand why one 'tense' or the other should be used. As you pointed out, those sentences are in certain sections of your book, and the only thing the author wants you to do is to produce the verb form that is the focus of that particular section.

1. It is theoretically possible to say "He will have been here before you go", however what that means is "He will have come and gone before you go". In other words, his "being here" will be finished at the time in the future when you go, and therefore he will no longer be here at that time.

A more commonly used construction would be "He will be here before you go." However, unlike the previous sentence (using the future perfect), this sentence means that he will be here at the time when you go.

2. Your second sentence works better with the future perfect: "The train will have left before we reach the station." In other words, the train will no longer be at the station at the time in the future when we arrive. The train's departure will already be complete/finished at that time in the future.
.