My final question about until / by

Question

I think in the sentences below, “by” doesn't work and the only proper preposition is “until”, because all of these sentences are negative where “by” doesn't make any sense at all.

-A- She won’t be back until 5 o’clock.
-B- He will not return until Monday.
-C- Why didn't you come until 12 o’clock?
-D- I won’t be ready until Tuesday.
-E- I can’t be ready until 8 p.m.
-F- You mustn't be at home until 7 o’clock.

Whereas If we change the negative forms of these sentences to positive, we will see that "by" would be the proper preposition and then "until" would not work. Am I right?

Answer 1

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.

Answer 2

This is a complicated question: it involves unstated implications and shifting ‘meanings’ which can only be sorted out by considering the aspect of the verbs involved.

First, the easy part.

Both by and until take as their object a point in time. I’ll call this T_terminal, because it is the endpoint of a timespan TS whose beginning is defined in your context. For instance:

She won’t be back until 5 o’clock. ... TS starts ‘now’, at the time of speaking.
Why didn’t you arrive until 12 o’clock? ... TS started (we may presume) at the point you started waiting for him.

Both by and until are used to locate an event in time. I’ll call this event E, and the time when it occurs T_E.

The first important difference between by and until is that they signify different locations in time for E:

• With by, it is asserted that T_E occurs during TS: ... T_E≤T_terminal.

  John will arrive by 5 o’clock. ... asserts that he will be at his destination at or before 5 o’clock.

  The negative asserts that T_E does not occur during TS:...T_E>T_terminal.

  John will not arrive by 5 o’clock. ... asserts that he will not be at his destination at 5 o’clock or at any time before that.

  Note that it is not asserted that he will arrive after 5 o’clock: he may never arrive.

• With until, it is implied that T_E occurs at the end of TS: ... T_E=T_terminal.

  John will not arrive until 5 o’clock. ... implies he will be at his destination at 5 o'clock.

  Note, however, that his arrival at 5 o'clock is not an entailment, a necessary understanding. The only entailment here is that John will not arrive before 5 o'clock. His arrival at 5 o’clock is an implicature, an understanding which may be ‘cancelled’ or qualified by further specification:

  John will not arrive until 5 o'clock, maybe later.

And now the hard part:— Why is this permitted only in the negative? Why can you employ the positive with by but not with until?

^okJohn will arrive by 5 o’clock.
^∗ John will arrive until 5 o’clock.

The answer lies in a consideration of verbal aspect—specifically, in the distinctive use of by and until with telic non-telic, and stative verbs.

• Telic (‘goal’) verbs are those such as learn or arrive which have a change of state ‘built in’ to their meanings.^† Non-telic verbs are a) statives, which name states (know, be), and b) activities (run, sleep)—these do not change but just keep on going until they stop.

Here is the other important difference between by and until:

• by marks E as a change of state within TS. When it is used with a telic verb, E is the culmination of that verb, the change of state it names:

  John will arrive by 5 o’clock. ... John’s journey will be complete at or before 5 o’clock.
  John will learn the results by 5 o’clock. ... *John

  When by is used with a nontelic verb, E marks the entry into the state named by that verb:

  John will know what to do by 5 o’clock. ... John will learn or figure out what to do at or before 5 o’clock.
  John will be here by 5 o’clock. ... John will arrive here at or before 5 o’clock.

  Note, however, that activity verbs resist this use. For instance, these uses would sound strange to most contemporary speakers:

  ^? John will sleep by 5 o’clock.
  ^? John will run by 5 o’clock.

  Our first inclination is to ‘recategorize’ sleep or run here in a telic sense: John will finish his allotted time of sleeping or running. To overcome this awkward sense, we cast the verb in the progressive mode, which recategorizes the verb as a stative:

  ^okJohn will be sleeping by 5 o’clock.
  ^okJohn will be running by 5 o’clock.

• until marks E as the end of a state which persists throughout TS. With activity verbs this is straightforward:

  John will run until 5 o’clock. ... John’s state of running will end at 5 o’clock.
  John will sleep until 5 o’clock. ... John’s nap will end at 5 o’clock.

  Many stative verbs are uncomfortable with until, because we think of the states they signify as more or less permanent—know, love, like. Others like have and be, accommodate until when they are understood to signify temporary states:

  John will have this room until 5 o’clock. ... After 5 o’clock you may use the room. John will be busy until 5 o’clock. ...After 5 o’clock he will be free to see you.

  Telic verbs denote events, not states, so they usually do not work with until when used positively. The above example, for instance, ^∗ John will arrive until 5 o’clock, only works if we can think of arrive as an activity, arriving repeatedly between now and 5 o’clock. That is extremely unlikely; but this would work:

  ^okJohn will arrive at 8 o'clock every morning until May.

  However, telic verbs can work with until when used negatively.

  ^okJohn will not arrive until 5 o’clock.

  This is because John is understood to be in a state of non-presence until he finally does arrive at 5 o’clock.

---

^† There are various sorts of telic verbs—accomplishment, achievement, semelfactive—but we won’t distinguish these.

Answer 3

It's ok to use both "by" and "until" in negative sentences, but these two words have different meanings. "By" indicates a deadline (and includes the deadline) for some action and "until" indicates a period of time where something is true that ends at a specific point. They are similar but they are not the same.

Both "by" and "until" are combined with a time phrase that means a point in time, for example:

by 8pm
until 8pm
by Friday
until Friday

If I say "This test must be completed by 8pm", that means that the deadline is 8pm for this test. You can finish the test at 8pm or any point before 8pm. "By" always includes the deadline time. Notice that this indicates an action, usually very short, which is completing the test.

If I say "He is giving a presentation at the theater until 8pm", that means that I can hear him talk the whole period before 8pm, but not at 8pm. Notice that this indicates a state (something that is true or false) or an action over a period of time, for example the presentation. "Until" does not include the end time, because this is when the state stops.

The meaning of "He is giving a presentation at the theater by 8pm" would be different. It means that at some point before or at 8pm this person will give a presentation, but not all the time in this period before 8pm. "Until 8pm" means the whole period before 8pm. Conversely, you can't say "This test must be completed until 8pm" because completing a test is a short action and doesn't make sense for it to happen over a long period of time, which is what "until" implies.

Does this make sense?

[Edit - Adding some points that were elaborated in comments.]

Answer 4

-A- She won’t be back until 5 o’clock. - YES

-B- He will not return until Monday. - YES

-C- Why didn't you come until 12 o’clock? - YES

-D- I won’t be ready until Tuesday. - YES

-E- I can’t be ready until 8 p.m. - YES

-F- You mustn't be at home until 7 o’clock. - YES

-G- I don't need the apartment to myself until 8PM. - YES

These all look good. However you can replace "until" with "by" in some of these and get a meaningful, grammatically correct sentence.

-A- She won’t be back by 5 o’clock. - YES

-B- He will not return by Monday. - MAYBE

-C- Why didn't you come by 12 o’clock? - MAYBE

-D- I won’t be ready by Tuesday. - MAYBE

-E- I can’t be ready by 8 p.m. - YES

-F- You must not be at home by 7 o’clock. - NO

-G- I don't need the apartment to myself by 8PM. - MAYBE

"By" and "until" have different meanings here. If the time in question is 5PM, "by 5PM" means 1PM, 2PM, 3PM, 4PM, and 5PM. "Until 5PM" means 5PM, 6PM, 7PM, 8PM, etc. This is why you can change "until" to "by" in some of these sentences and they still make sense. Not all of them... the ones marked MAYBE seem a bit illogical if you use "by", and the one marked NO is a negative command so you cannot use "by". But the ones marked "yes" can definitely be changed. They just have a different meaning.

Now let's take a look at the positive forms of these sentences...

-A- She will be back by 5 o’clock. - YES

-B- He will return by Monday. - MAYBE

-C- Why did you come by 12 o’clock? - NO

-D- I will be ready by Tuesday. - MAYBE

-E- I can be ready by 8 p.m. - YES

-F- You must be at home by 7 o’clock. - YES

-G- I need the apartment to myself by 8PM. - YES

Some of the sentences above would sound much better with a preposition that is neither "until" nor "by". You want to turn them into statements of fact rather than using periods of time.

-A- She will be back at 5 o’clock. - YES

-B- He will return on Monday. - YES

-C- Why did you come at 12 o’clock? - YES

-D- I will be ready on Tuesday. - YES

-E- I can be ready at 8 p.m. - YES

-F- You must be home at 7 o’clock. - YES

-G- I need the apartment to myself at 8PM. - YES

Finally, let's take a look at some positive sentences used with the preposition "until".

-A- She will be back until 5 o’clock. - NO

-B- He will return until Monday. - NO

-C- Why did you come until 12 o’clock? - NO

-D- I will be ready until Tuesday. - NO

-E- I can be ready until 8 p.m. - NO

-F- You must be at home until 7 o’clock. - NO

-G- I need the apartment to myself until 8PM. - YES

Most of these constructions are not allowed, but as you can see -G- is an exception.

This is a complicated subject and I do not have the grammar rules memorized. Hopefully these examples are helpful, and somebody else who knows the grammar rules can also post an answer.