Traditional grammar classifies sentences as simple, complex and compound. But this scheme conflates two separate dimensions: the presence or absence of embedding, and the presence or absence of coordination. Consider:
(1) 'Jill seems quite friendly'.
(2) 'I think Jill seems quite friendly'.
(3) 'Jill seems quite friendly but her husband is extremely shy'.
(4) 'I think Jill seems quite friendly but her husband is extremely shy'.
(Note that in (4) there is both embedding and coordination.)
We can distinguish (1) and (2) from (3) as non-compound (or clausal) vs compound (a coordination of clauses); (1)-(2) could then be distinguished as simple vs complex clauses but no great significance attaches to this latter distinction.
The crucial point is that sentences are more productively categorised into "type":
Declarative (You are very tactful)
Exclamative (How tactful you are!)
Imperative (Be very tactful)
Interrogative (Open: How tactful are you? ~ Closed: Are you very tactful?)
Within those categories there are a number of sub-types such as main, subordinate, finite (content, relative and comparative), non-finite (infinitival, gerund-participial, past participial) and so on.
Thank you for responding. I see what you mean. Grammar can be very confusing with all the different clashing of terms that exist. Also, there are disagreements among grammarians or grammar experts about usages and definitions. So it is difficult to know who are correct on these points. That has somewhat been my struggle on this. Well, that is a part of learning.