For/to ?

[pidr1nhu]

I really get embarassed using these words, I believe that it's easy to memorize the common phrases that contains them, then trying to apply them in a phrase.
What's the difference, what aspect do I use to distinguish them?

And another slight question! How do I know when it's needed to double the consonant after adding ING to the end of a verb.
Example:
Stopping

[Casiopea]

Prepositions for and to: Could you give us a few example sentences?

Spelling: click here and practise.