Hi !

As a rule, you should try not to end a sentence with a Preposition (in, on, at, by, for, to, with etc.),

so do not write or say :

“When I leave the office, I turn my computer off”,

but rather :

“When I leave the office, I turn off my computer”.

I recently heard a story in which an intelligent and smart Afro-American from a poor neighbourhood

was accepted to the prestigious University of Harvard.

On his first day, he approached two more typical Harvard students and asked them, in his rather ghetto-style English :

“Say, do you know where the library is at ?”

One of them answered in a very patronizing way :

“Here at Harvard we do not end our sentences with prepositions.”

The new Afro-American student thought for a couple of seconds and then asked again,

this time making sure the sentence did not end with a preposition :

“Say, do you know where the library is at, asshole ?”

Moral : If you are going to break the rule, at least do it with style 🙂