Sentence structure is about how words, phrases, and clauses fit together to form a complete thought. Every sentence needs at least a subject and a verb.
The four basic types: simple (one independent clause, “The dog barked.”), compound (two independent clauses joined by a conjunction, “The dog barked, and the cat ran.”), complex (an independent clause plus a dependent clause, “The dog barked because it heard a noise.”), and compound-complex (a mix of both, “The dog barked because it heard a noise, and the cat ran.”).
Quick practice: identify the type in “Although it was raining, we went outside, and we got soaked.” (Answer: compound-complex.)