We use a declarative sentence to make a statement. A statement usually gives us information. Begin it with a capital letter and end it with a period (.).
The koala bear is eating leaves.
Gavin likes to surf during the summer.
Cicilia is reading under the tree in the sunshine.
We use an interrogative sentence to ask a question. Begin it with a capital letter and end it with a question mark (?).
Do you know when the first day of fall is?
Have you been to the new art gallery?
Can you tell me where the nearest subway station