Use awhile as an adverb meaning for a while. Use a while as a noun phrase meaning a period of time. A simple rule from multiple usage guides is this: awhile usually modifies a verb directly, while a while often appears after a preposition such as for, in, or after.

✅ Stay awhile.
Here, awhile means for a while and modifies the verb stay directly. Merriam-Webster and Grammarly both describe awhile this way.

✅ Stay for a while.
Here, a while means a period of time and follows the preposition for. MLA and Merriam-Webster both note this pattern.

✅ I haven’t seen her in a while.
Because the phrase follows the preposition in, the two-word form is the standard choice.

❌ I haven’t seen her in awhile.
More standard: ✅ I haven’t seen her in a while.
Usage guides consistently note that awhile does not normally follow a preposition.

The most common mistake is using awhile after a preposition. That is where writers most often get tripped up. Traditional usage guidance says awhile works on its own as an adverb, but a while is the form you want after for, in, after, and similar structures. Merriam-Webster does note that the two forms are increasingly used interchangeably in real life, but it also says the traditional distinction is still commonly observed.

Use this shortcut:

So:

That is the core pattern described by MLA, Merriam-Webster, and Grammarly.