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When you warn to show the means or the instrument with which the action is done, use with. By denotes the order of the action: The bird was shot by the man.
The following take by and not with: by hand, by post, by phone, by one's watch, by the hour, by the dozen, by the metre.
Use on with the days of the week or month: on Friday, on March 25, on New Year's Day. Use at with the exact time: at four o'clock, at dawn, at noon, at sunset, at midnight. Use in with a period of time: in April, in winter, in 1945, in the morning. Also at night and by day.
Don't use for in the sense of about. The chief use of for is to convey the idea of being in favour of. If we say that the teacher spoke for bad habits it's like saying that he/she spoke in favour of bad habits!
We use in to describe the physical location of something as part of a larger thing or place. We use at when we're talking about an address, a public place or building (a bus stop, the Post Office, the library etc.) and cases in which the location is irrelevant but what we do there is what matters (school, the dentist, dance class etc.)