From my understanding, "Bob stayed indoors" is a correct expression, while "Bob stayed indoor" is incorrect. Why is the latter incorrect? The term indoor is an adjective, and the verb stay is often followed by an adjective (e.g., "Let's stay focused.")
The verb STAY creates a space for a Subject and a Predicative Complement:
- The elephants stayed awake.
Here the Subject is the phrase the elephants and the Predicative Complement is the word awake. A Predicative Complement usually describes the Subject or Object of a verb. With the verb STAY the Predicative Complement describes the Subject. So, in this example, awake describes the elephants.
Predicative Complements can be noun phrases, adjective phrases or preposition phrases:
- The elephants stayed friends. (noun phrase)
- The elephants stayed vigilant. (adjective phrase)
- The elephants stayed in the kitchen. (preposition phrase)
Predicative Complements cannot normally be adverb phrases:
- *The elephants stayed largely. (Ungrammatical)
- *That woman is beautifully. (Ungrammatical)
These sentences are ungrammatical if the adverbs are understood a Predicative Complements.
The Original Poster's Example
The word indoor can be considered an adjective. However, it is an attributive only adjective. We can only use it before nouns. We cannot use it as a Predicative Complement:
The elephants want an indoor swimming pool.
*The elephant was indoor. (ungrammatical)
Because we are making a sentence with the verb STAY, we need a word or phrase that can function as a Predicative Complement. We cannot use the attributive adjective indoor. But we can, of course, use the preposition indoors. This word looks very similar to the adjective, but it is a preposition:
- The elephants stayed indoors.
- Bob stayed indoors.
Grammar reference Teachers and very high level students: you can read about indoors and other prepositions like this that occur without following nouns in The Cambridge Grammar of the English Language.
Indoor is an adjective, but is only ever used attributively (preceding a noun phrase), not predicatively (following a verb). Most adjectives can be used either way, but not all. Another example of an adjective which cannot be used predicatively is former.
Indoors is an adverb (of place), and can follow a verb like stay.