Declarative and interrogative are the two main sentence types in English; the others are exclamative and imperative. There are two kinds of interrogative: closed and open, so there are five sentence types altogether:
1. Declarative: Used for making a statement, for example "You are very careful".
2. Closed interrogative: Used for asking a closed question, for example "Are you very careful"?
3. Open interrogative: Used for asking an open question, for example "How careful are you"?
4. Exclamative: Used for making an exclamatory statement, for example "How careful you are"!
5. Imperative: Used for issuing a directive, for example "Be very careful".
Note that the closed question gets its name from the fact that there are a closed set of answers. For example, in "Are you very careful"?, there are just two answers, "Yes, I am careful" or "No, I am not careful".
On the other hand, in "How careful are you"?, the set of answers is open-ended. The reply might be "Well, most of the time, I'm careful" or perhaps "It depends on what I'm doing", or one of many other similar answers.