Complicated sentences and Complex sentences

What is a complex sentence?
A complex sentence is a sentence which has one independent clause with one or more dependent clauses, which, unlike both clauses in a compound sentence, CANNOT stand alone.

What are the different parts that make up a complex sentence?
Two subjects and verbs (one for each clause), a subordinator (kind of like FANBOYS, joins the two clauses together, FX. Because, Since, After...), and if the subordinator is a the beginning of the sentence, a comma in between the two clauses.

What are the differences between complex sentences and compound sentences?
Compound sentences have two independent clauses that can both stand alone in a sentence. Complex sentences have one independent clause and one dependent clause, which cannot stand alone. Also, Compound sentences are linked with FANBOYS, complex sentences can have many other subordinators.

I went to Social Studies after lunch, but my friend headed off to RLA. While I was working on my Social Studies project, I realized I had forgotten my computer. I ran down to the RLA room, but my friend wasn't in his class. I did not see my friend because he had left to find me. Although I had forgotten my computer at the cafeteria, my friend had already found it and was trying to return it!