What Are The Rules For Using Prepositions For And By?

When used as a preposition rather than a conjunction, the word "for" is used to infer destination, service or purpose toward, e.g., "She went to work for that company".

The word "by" most often indicates a position reference (as in "She left her keys by the door"), but may also be used to show a means ("She finished the assignment by working night and day on it", or  to show a margin or distance (Secretariat won the race by a nose).

It is almost always incorrect to end a sentence with the preposition 'for'  (Incorrect:  What did you do that for?  Say instead:  "Why did you do that?"). But like any good grammar, rule, there are exceptions, and it is acceptable to place "for" at the end of the sentence in instances another word is dependent upon placement of the word "for" as a prepositional phrase unto itself.  (I know, that sounds complicated, but an example simplifies the idea:  We can say "That was uncalled for" but it wouldn't make sense to say " that for which was uncalled".  The phrase 'uncalled for' is actually working as an adjective in this case.)

It is acceptable to end a sentence with the preposition 'by', however:  "They sat for a time and watched the people walking by". (Here, we see 'by' is nearly serving the purpose of an adverb as it is used to describe where or how people are walking.)

If you need a more in-depth explanation or information, here is a site I have found helpful in the past.  The site doesn't just list endless rules; it provides context and explanation for most common grammar errors and usage.  A quick search yields good information on almost every topic. Here it is: www.askoxford.com