What does "only" mean? I understand the technical and logical distinction between "if" and "only if" and "if and only if".  But I have always been troubled by the phrase "only if" even though I am able to parse and interpret it.  Also in my posts on this and other sites I have frequently had to make edits to migrate the term "only", sometimes across multiple structural boundaries of a sentence, which is empirical evidence to myself that I don't intuitively know the meaning of the word. 
Is there any simple rule that I can use to determine whether or not it is appropriate to use this word in a particular context in order to achieve more clarity?  In mathematical discourse, what are some other common lexical contexts, meaningful or not, in which appears the word "only"?  Why do I often write "only" in the wrong place?

I think analogies in plain English are the way to internalize this... so here's one:
Given that you want to wear socks with your shoes, put your shoes on only if you have already put your socks on.
The idea is that there is no other way to arrive at the state of having your socks and shoes on (aside from the ridiculous possibility of placing your socks over your shoes).
