Classifying sentences is easiest if you mark the parts and then see what you have. Underline independent clauses once. Underline dependent clauses twice.
1 independent = simple
2 independent = compound
1 independent and 1(or more) dependent = complex
2(or more) independent and 1(or more) dependent = compound-complex
We might go to New Orleans on our Christmas vacation or to New York next spring.
(1 independent = simple)
Our families have been friends forever, but we haven’t seen each other in years.
(2 independent = compound)
When we get to your aunt’s house, you must remember to thank her for the presents that she sent you.
(1 independent + 2 dependent = complex)
Before we go to the movie, which you have been waiting weeks to see, we need to find a babysitter for your little sister, who is too young to see it.
(1 independent + 3 dependent = complex)
The Egyptians who built the pyramids were amazing architects, but the Romans who built many years later were even better because they were able to build arches, which the Egyptians hadn’t figured out.
(2 independent + 4 dependent = compound-complex)