Is the sentence "I was sitting alone." correct? And if so, is "alone" an adverb? Are there other examples of adjectives being used as adverbs without modification?
Take the sentence into the simple past and it becomes 'I sat alone.'
Clearly, here, 'alone' modifies 'sat'. As such, it is an adverb.
If you were to say 'I, alone.' you could consider it an adjective. Others might argue differently. If the sense, however, was 'I was alone', then 'alone' would would arguably be a subjective complement.
The easiest way to think of adverbs is in relation to the 9 attributes which are part of Aristotle's 10 categories of being (everything except substance), in which terms they can simply be classified as 'attributes of attributes'.
I agree with Barrie's answer and would say there are a lot of adjectives that can be used as adverbs. English dictionaries usually indicate uses of adjectives as adverbs simply as "adverbs".
I don't like this change of word classes. And I think this does not help to understand language. I prefer the formulation "adjective in adverb position".
In the sentence given in post 1 "I was sitting alone" alone is not an adverb, even when dictionaries say so. The basic concept is: I was alone - and that is an adjective. Even if you replace "was" by "was sitting" or "was standing" that does not change anything. "was sitting" is can be considered as a variant of the verb to be (the basic copula verb, that has a lot of variants).
Another example: The birds were flying high/low. Dictionaries list this use under high/low adverb. But that is really confusion of things. The bird fly at a high/low level and not in a high/low manner. "high/low" maintain their adjective character and the speaker has a clear concept of an adjective use in mind: at a high or low level.
Such adjective uses must be studied carefully and sometimes it is not so easy to decide how things are. In John Wyndham's excellent science fiction story The Meteor he describes the landing of a small extraterrestian space craft on Earth. He said: The thing burried deep into the earth. Here you might have difficulty to decide what would be right: deep or deeply. But Wyndham decided to take the adjective deep, obviously thinking the flying object burried to a deep point in the earth.
The word “alone” didn’t describe the "I". thus it is not a adjective. It tells a condition around the I, not the I itself. "Alone" didn’t describes how I sat. It seems like a different category, because sit is a action, and alone is condition. Thus it is also not a adverb. This sentence is combined with two sentence: “I sat.”and “There is no people around me.”It is a combined sentence being simplified.