Question

Since we had only gone a mile from camp, we could turn back before dark. 
Is this sentence a simple, compound or a complex sentence?

Answer 1

it is a complex sentence.

Answer 2

Since we had only gone a mile from camp, we could turn back before dark.

This sentence is in a complex form.

1. A complex sentence can be defined as a sentence that has only a single unconventional clause and one conventional clause. This further defines that the terms which are used are not exactly matching with each other, they often result in coordinating conjunction that can further switch the level of the order of one or multiple terms to make their equality go down.

For example- He practiced that art all day and night hard so that he get good results in the morning.

2. Another example of converting a simple sentence into a complex sentence-

He denied his words. (Simple sentence)

He has denied what he said. (Complex sentence)