Many people wrongly believe that till is an abbreviation of until and say it should not be used. Traditionally, till is the older and original form, coming from Old Norse til into northern Old English. The form until is a later compound of Old Norse und (meaning as far as) + till, and originated in northern Middle English:
She’ll be in the store until 9 pm. She’ll be in the store till 9 pm.
Is second sentence is wrong? No, until and till are both real and correct words and mean the same thing. They denote how long something will happen or when it will start or end. Actually, till is the older of the two words and you can still use till in formal writing without being wrong: The neighbours quarrelled from noon till night. (sometimes the single syllable of till is better for the rhythm of your sentence than the two syllables of until; here, till is informal, even spoken English)
Until denotes when something will happen, begin, or end: she has to work in the store until 9 pm; you have to hand in the report any day until Saturday; the market is open until 8 pm.
Notice that till is not a contraction of until — it’s actually older than until — hence, it should not be written with an apostrophe. ’Til shows up every so often, but many major usage dictionaries and style guides regard it an error, so it’s best to avoid it.
In contrast, as a preposition of time, by denotes ‘before or not later than a particular time’; by 11:30 am; arrive by Monday; I’ll be done by twelve o’clock.
Let’s take a look at how and when we should use each of these prepositions:
I’ve got to do this report until Thursday. (begin to do this report now, continue to do it until Thursday, then stop doing it regardless of whether it is finished or not)
I’ve got to do this report by Thursday. (make sure that on Thursday the report is done)
We can finish this project until Monday. (Now do this project on Monday: we’ll stop even if we haven’t finished)
We can finish this project by Monday. (Now do this project and finish it on Monday (it will be finished)
The train can’t leave until 5 am. (it is impossible for train to leave before 5 am)
The train must leave by 5 am. (train needs to leave any time before 5 am. If it leaves after 5 am, then there will be some sort of problem or trouble)
Therefore, until refers to all the time between now and the end of something while by only refers to the deadline.
Use until when you refer to the period of time before a deadline.
Use by when you refer to a deadline.
More example sentences of until
I’ll look after the children until you come back. (as conjunction)
They have to wait until the train arrives.
Mother won’t start cooking until she comes home. (a negative construction (with not) in the main clause)
She can’t mend the clothes until the sewing machine is fixed.
She can’t mend the clothes until the sewing machine has been fixed. (also have + past participle and had + past participle)
She didn’t serve the dessert until we had finished eating.
They’ll wait until Tuesday. (as preposition)
She’ll be at the airport until seven o’clock.
She partied with her friends until it got dark.
We’re not allowed to go out until we finish our dinner. (or until we have finished our dinner)
We didn’t know he was a foreigner until he spoke.
We travelled together until Boston.
He was coughing and stayed awake until midnight.
Notice that with until we usually use the words can, will, or any present tense verbs.
More example sentences of by
The project needs to be ready by next week. (before or not later than a particular time; Indicating a deadline or the end of a particular time period)
By the end of this month the organisers had sold all tickets.
By the time she got home she was tired.
I’ll be home by 11, I promise.
By 8.00, most of the guests had gone.
Please try to have this project done by Sunday.
You had promised to be back by seven o’clock.
The documents must be in by the 15th to be accepted.
By the time I got to the airport the entry had already been barred.
Notice that we usually use the words have/had, need, or must when we use by.