A declarative sentence usually makes a statement. It can report a fact, express an opinion, or describe a situation. In The library opens at nine, the speaker states when the library opens.
Declarative word order
In a typical declarative clause, the subject comes before the verb. In The library opens at nine, the library is the subject and opens is the verb. The library opens at nine is affirmative, and The library doesn’t open at nine is negative. Both use declarative word order.
Punctuation and purpose
A written declarative statement normally ends with a period. The same word order can ask for confirmation in dialogue, as in The library opens at nine? The usual interrogative form would be Does the library open at nine?, with the auxiliary does before the subject.
