Question

Be so kind as to feed my dog.

Can this be an interrogative sentence?
im bit confused.

Answer 1

It is interrogative only if preceded by "Would you.." The latter form would also be much more polite. "Be so kind as to.. " can be considered a rather way of giving an order order, depending upon the tone of voice and body language.

Answer 2

English sentences essentially fall into four categories:

declarative -- normal sentence, giving information, stating facts, etc.

interrogative -- asking a question

imperative -- giving a command

exclamatory -- sudden, strong statement, but may be missing subject or verb

Which one do you think you have here?

??