Learn English – Tag question of negative form

A sentence becomes negative by specific words. Most of those have 'no' in it. For example

No
Nobody
No one
Never
Not
Nowhere
...

And yes a sentence without a subject can be negative. Here's an example:

Stop the car!  -- positive
Do not stop the car! -- negative

Do your homework!
Don't do drugs! 

In these cases, 'by' can be used but it changes what the speaker is expressing.

She won't be back until 5 o'clock.

expresses that Penelope will return at exactly 5 o'clock.

She won't be back by 5 o'clock

expresses that Penelope will return sometime after 5 o'clock. There is also an implication that something else will happen at 5 o'clock that Penelope will miss.

She won't be back until after 5 o'clock.

expresses that Penelope will return sometime after 5 o'clock, but nothing else is happening for Penelope to miss.

She will be back until 5 o'clock.

expresses that Penelope was away, then returned, is here now and will leave again at 5 o'clock.

She will be back by 5 o'clock.

expresses that Penelope will return sometime before 5 o'clock.