A while and awhile sound the same, but they are not interchangeable in every sentence.
A while is a two word phrase. It acts like a noun phrase meaning a period of time. It often appears after a preposition such as for, in, or after. Example: We waited for a while.
Awhile is a one word adverb. It means for a short time and usually works without a preposition before it. Example: Please sit awhile.
- Use a while after a preposition: She rested for a while.
- Use awhile when it stands alone as an adverb: She rested awhile.
A simple check can help. If the sentence needs for, use a while. If you can remove for and the sentence still works, awhile may be the right choice.