Consider these sentences:
They are good.
They are working.
They are eating lunch.
In first sentence
are is a linking verb, and
good is an adjective; so it has the form
S+V+C (Subject + Verb + Complement).
I have a difficulty figuring out the roles of
working in the second sentence. Clearly
working is not an adjective nor is it a noun, so it cannot be a subject complement. Any help?
I think the last sentence is pretty straightforward. Here
are is helping verb,
eating is the main verb, and lunch is the object; so it has the form
S + V + O (Subject + Verb + Object).