What's the key to success?
What's the key for success?
Which one of the 2 sentences above is correct, and why?
Which one of the 2 sentences above is correct, and why?
Both are correct. How? I would try to explain with a brief example for both:
What's the key for success?
This specifies success as a door. So, this question/statement asks for a key to that door.
What's the key to success?
This specifies success as something on the other side of the door. So, this asks for a key of that door to get to the other side, which is success.
So, both makes equal sense. However, the latter one: What's the key to success? is used more often.
The first one is correct and what you are looking to say.
What's the key to success?
Meaning what is the one thing that is crucial to success. This is what you should use.
However...
What's the key for success?
While is sort of scans it's poor English. A more commonly used phrase would be to say What's the recipe for success?. Which has a different meaning because it implies there's more than one thing needed for success.
According to Ngram "the key to success" is preferred, "the key for success" is hardly used. Link
I would say "the key to success" is analogue to "the key to the door".
What's the key to success?sounds better to me. Perhapskey to successis an idiom, or perhaps there is a grammatical reason for it. I'll let somebody else try to explain the grammar ; ) – RedDragonWebDesign Apr 6 '16 at 7:45