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The preposition ‘by’ used for time communicates a very specific idea. When we use it for time, it communicates the idea of ‘some time before’ and ‘no later than.’ A classic example: when a child asks their parents if they can go outside to play with their friends in the afternoon, it’s common for the parents to say something like: “Ok, but you need to be back home by six o’clock.” his means that the child must return home no later than six o’clock; six o’clock is the absolute latest time that the child can return home. If they return home later than six, they will be in trouble.
Another classic example of the same idea is a teacher telling their students when they need to finish a project, a composition, or homework, for example. If the teacher tells their students on Monday, “I need to have your compositions on my desk by Friday,” they mean that the students can turn in their compositions on Tuesday, Wednesday, Thursday, or even Friday, but not Saturday; Saturday would be too late. This sentence means that Friday is the absolute latest time that the students can submit their compositions; it is the deadline.
We see something in common with these two examples: when we use the word ‘by’ on its own, we say a specific time, day, month, year, etc. We say: by noon, by next Monday, by June, by 2025, by this time next month, etc. Again, it means that the time mentioned is the deadline: a certain action must be performed or something will happen at some point before that specific mentioned time.
There is another situation in which we use the preposition ‘by’ to talk about time, and it is the phrase ‘by the time.’ We use this phrase when we don’t know the precise time at which something will happen, so we need to give more information. Imagine a father is going to go to the supermarket, and before he leaves he tells his children that they must clean the house at some point before he gets back home. In this situation, the father obviously doesn’t know at what time he will return, therefore he can’t give a specific time (‘by three o’clock’, for example). In this case we have to use the phrase ‘by the time.’ Directly after this phrase, we put a subject and a verb. The father can say: “You need to clean the house by the time I get home.” Some other examples of this are:
- The students will finish their exercises by the time the lesson ends.
- By the time we arrive at the station, our train will have left!
- By the time they realize, it’ll be too late.
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