Explanation

Four sentence types by purpose: declarative (statement, ends with period), interrogative (question, ends with question mark), imperative (command), exclamatory (strong emotion, ends with exclamation).

Four types by structure: simple (one independent clause), compound (two independent clauses joined), complex (independent + dependent clause), compound-complex (combination).

Varying sentence types makes writing more interesting. Too many simple sentences sounds choppy. Too many complex sentences becomes hard to follow. Mix them.