~てからでなければ vs ～なければ


I'm learning the ～てからでなければ... grammar pattern, which my textbook defines as:

[code]

    Until/unless ~ happens or is done, ... cannot happen or be done either. Used in negating or negative statements.
    
[/code]

One of the practice problems is as follows (pick the best answer):

[code]

    ４時に（　　）飛行機の時間には間に合わない。
    
[/code]

A: 起きなければ

or

B: 起きてからでなければ

The answer key says A is correct, but I don't understand why. Is there
something grammatically wrong with B or is there a semantic nuance difference?




> Until/unless ~ happens or is done, ... cannot happen or be done either. Used
> in negating or negative statements.

I agree with the definition of ～てからでなければ in this sentence.

**４時に起きてからでなければ飛行機の時間には間に合わない。**

_Until/unless you wake up at 4 o'clock, you cannot arrive the time of
departure of flight._

This is a weird situation isn't it? If you wake up before 4 o'clock say at 3
o'clock, will you miss the flight?

**４時に起きなければ飛行機の時間には間に合わない。**

So, this is conditional sentence.

_If you don't wake up at 4 o'clock, you will miss the flight. ⇨ If you wake up
before 4 o'clock, you can get on the flight._




> Until/unless ~ happens or is done, ... cannot happen or be done either. Used
> in negating or negative statements.

I agree with the definition of ～てからでなければ in this sentence.

**４時に起きてからでなければ飛行機の時間には間に合わない。**

_Until/unless you wake up at 4 o'clock, you cannot arrive the time of
departure of flight._

This is a weird situation isn't it? If you wake up before 4 o'clock say at 3
o'clock, will you miss the flight?

**４時に起きなければ飛行機の時間には間に合わない。**

So, this is conditional sentence.

_If you don't wake up at 4 o'clock, you will miss the flight. ⇨ If you wake up
before 4 o'clock, you can get on the flight._

