By the time & When

[crazyaboutenglish]

Hi everyone, my teacher told me sentence one and two are correct and mean the same but that three is not correct and four is correct.

Can anyone tell me why 3 is not correct?

1. By the time James retires I will be fifty-nine.
2. When James retires, I will be fifty-nine.

3. By the time I leave here, I will do a demonstration in the office.
4. When I leave here, I will do a demonstration in the office.

Thanks!

[Grumpy]

1. "By the time..." something happens [ie James retires] must relate to an event that will have already happened or will be in place at the specified time, ie "I will be fifty-nine".

3. "By the time I leave here, I will do..." means that the demonstration will not yet have been done, so the phrase is incorrect. However, if it was expressed as "....I will have done a demonstration in the office", that would be correct.

"When", on the other hand, can mean things what have already happened and things that will happen in the future, so 2. and 4. are both correct.

[crazyaboutenglish]

THANK YOU!

[Rover_KE]

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Rover