It is a colloquialism, an informal way of saying "What can I do for you", or "How can I help you"? "What can I do you for" is fine for causal language, but it would not be considered tasteful or polite in some settings.
Yes. It cleverly suggests correct use and wrongful use.
It depends how you use it. If you use it after something it can be correct. But being in a sentence by itself isn't correct.
No it is not correct.
The Correct Use of Soap was created in 1980-05.
"Tomorrow's weather is meant to be fine" is a correct way to use it.
Use is present. Used is past. The correct sentence is, This is used for....
"Did you have your breakfast" is the better one to use.
The correct spelling is "laid".
A balance is the correct tool to use to find the mass of an object.
Both are correct.
If you use 'you and he' as the subject of the sentence, it is correct: You and he will meet when we get to the restaurant.
I think 'on foot' is the correct term to use.