adverbs - “By the time” = “Before”?

By the time he comes, we will have already left.

Have I correctly reworded the sentence above?

Before he comes, we will have already left.

Answer

It's a little easier to talk about if we ground the sentences just a little more in terms of intended semantics. The following (which is not absolutely required) doesn't change the discussion at all, but may make it easier to talk about:

By the time he comes (here), we will have already left. Before he comes (here), we will have already left.

According to Mac Millan Dictionary,

• by the time: used for saying what has already happened at the time that something else happens. By the time we arrived, the other guests were already there.

However, I have seen two usages of "by the time":

1. By the time X, Y" is typically used to mean that event Y occurs before X. Example: "By the time we turned on the TV, the movie had (already) started". This means we missed some of the beginning of the movie.


2. I've seen "by the time" to mean "not later than" when used as a prescription, like a contract from a teacher to a student: "You must be in your seat by the time of the bell or you will be sent to the principle." In that case, if the student is sitting down at exactly the same time as the bell, then he may get a disapproving look from the teacher, but will not get sent to the principle.

Based on the idiom you are using - "By the time he comes (here), we will have left" your sentence fits the first definition. It would be absurd that you could leave (and miss meeting him) at exactly the same time he arrives. Your sentence is further semantically coerced to fit definition #1 by the use of "already".

However, there's a bit of ambiguity in the word "comes", which is not enough to indicate a specific point in time. This is not necessarily a problem, but something to be aware of. Without any context, it's unknown if your word "comes" means "departs" or "somewhere on the way" or "arrives". But that doesn't matter because you'll be gone regardless. I can say, "Before he comes (here), make sure he has his wallet" and I'm referring to a point in time before he even leaves his house. Or I can say, "before he comes (here), have him pick up some ice" and I'm referring to his arrival, so he must pick up the ice after he leaves his house but before he gets to my house. Or I can say, "he came to the party early" and I'm referring to the time he arrived.

You could reword as follows:

• Before he comes (here), we will have already left. (Leave it ambiguous - it might be clarified by context.)


• Before he leaves (from his place), we will have already left.


• Before he arrives (here), we will have already left.


• Before he gets here, we will have already left. (More informal and conversational.)


• We will have already left before he arrives.


• We will have left some time before he arrives.



• He will arrive (here) after we have left.


• If he departs now, he won't make it here before we leave.