How careless of you to forget the soap is that a declarative sentence?

No, the sentence "Careless of you to forget the soap, is that a declarative sentence?" is not a declarative sentence.

Here's why:

* Declarative sentences make a statement. They state a fact or opinion. This sentence is asking a question.

* Interrogative sentences ask a question. This sentence ends with a question mark, which is a key characteristic of interrogative sentences.

Therefore, the sentence is an interrogative sentence, not a declarative sentence.