The front door slid open.
I am curious about "slid open" in this context. Is "slid" a verb or an adverb? Is "open" a verb or not?
Is a "slide open" a phrasal verb like "push open"?
I wouldn't call it a phrasal verb. In phrasal verbs, the the two words together take on an entirely new meaning. For example, when I say:
I don't mean, "Why don't you grow taller?"; I mean, "Why don't you act more mature?"
However, when I say:
I mean exactly what the two words mean: open the door, by sliding it.
The next part of your question is interesting. My first thought was that slide is the verb and open is an adverb:
But that last one seems a bit peculiar. It seems like I am trying to say:
Which made me wonder if open is the verb and slide is the adverb.
Back to your sentence:
I think slid is the verb in that sentence. Why? Because that's the word I would change when I'm changing the tense of the sentence:
The word open functions as an adverb – we can confirm this by replacing open with other adverbs: