“Awhile” is used as an adverb. “A while” is used as a noun.

Here are some examples for how they should be used:
We have a while left before the train arrives.
I saw my friend a while ago.
We waited awhile for the bus.
She ran awhile.

Here’s a trick that usually works: If you can use “for a while” as a substitute, then using “awhile” will likely be the right choice when choosing between, “a while” and “awhile.”

Machine Briefs:
awhile= A/WHAOIL
a while= AEU/WHAOIL