Unless

Question

Hello. Which is correct?1) You can't do it unless you are back by 5. (Pr. Sim.)2) You can't do it unless you have been back by 5.(Pr. Perf.)

Terryxpress · Accepted Answer

What do you think is the most important word in your sentences that determines the tense?

Veronica 222 · Answer

'By' is.

Terryxpress · Answer

Yes...and what tense is 'can't"?

Veronica 222 · Answer

Can' means Future Simple Tense here, I think

Terryxpress · Answer

Aah!
Let's sort that out: "I can't/ I cannot..." That is, 'can' and 'not'.
So 'can't' is Present Tense.
How does that fit with the Present Perfect tense in your second sentence?

Veronica 222 · Answer

My teacher said to me that I can also use Present Perfect. (When we have 'by' we have to use Future Perfect)

Terryxpress · Answer

Hmmmmmmmm.
What is the meaning of 'by' ?

Veronica 222 · Answer

F.E:I will do itby 6 o'clock ('by' means close to time)

LouiST · Answer

TerryxpressWhat do you think is the most important word in your sentences that determines the tense?

'by' doesn't determine the tense in this sentence. You're wrong.
The most important here is: It's conditional. (here: unless)
And there is no conditional like:
'present tense', + 'if/unless/when' + 'present perfect'. This doesn't exist.

You can't do it, unless you have been back by 5.
-> Doesn't work.
Incorrect

You can't do it, unless you are back by 5.
-> zero conditional
OK

If you don't want to say it that strong:
You won't be able to do it, unless you are back by.
-> First conditional
OK

By the way, a comma is needed.
LS!

CalifJim · Answer

Veronica 222Hello. Which is correct?
1) You can't do it unless you are back by 5. (Pr. Sim.)
2) You can't do it unless you have been back by 5.(Pr. Perf.)

Recall that an asterisk ( * ) prefixed to a sentence is the standard way of indicating that the sentence is not grammatical.

Dynamic verbs:
You can't do it unless you return by 5.
You can't do it unless you have returned by 5.
You can't do it unless you wash the car by 5.
You can't do it unless you have washed the car by 5.
You can't do it unless you find the keys by 5.
You can't do it unless you have found the keys by 5.
You can't do it unless you finish your homework by 5.
You can't do it unless you have finished your homework by 5.

Stative verbs:
You can't do it unless you are ready by 5.
*You can't do it unless you have been ready by 5.
You can't do it unless you are back by 5.
*You can't do it unless you have been back by 5.
You can't do it unless you have the complete list by 5.
*You can't do it unless you have had the complete list by 5.
You can't do it unless you know all the irregular verbs of English by 5.
*You can't do it unless you have known all the irregular verbs of English by 5.

If you have trouble seeing the pattern, just ask for further details.

CJ

Unless

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