A complex sentence has two parts: a subordinate clause and an independent clause.
The subordinate clause begins with a subordinating conjunction and must be joined to an independent clause.
The word although is a subordinating conjunction.
If you begin a sentence with the word although, you are writing a subordinate clause.
Although it is sunny today
This subordinate clause cannot stand alone. It doesn’t make sense until we add an independent clause to finish the complex sentence.
Although it is sunny today, the wind makes the air feel very cold.
Here are some subordinate clauses that start with although:
Although I run fast
Although we have school tomorrow
Although I like cookies and cake
Notice that each of the subordinate clauses sounds unfinished. We must add an independent clause so that each sentence makes sense. Here are some complex sentences beginning with although:
Although I run fast, my sister is even faster.
Although we have school tomorrow, we only have to attend for three hours.
Although I like cookies and cake, my favorite dessert is ice cream.
The subordinating conjunction though is a shortened version of although. Use though the same way you would use although:
Though I run fast, my sister is even faster.
Though I like cookies and cake, my favorite dessert is ice cream.
Anytime you begin a sentence with the word although or though, you are writing a subordinate clause. You must put a comma after the subordinate clause and then add an independent clause to order to make a complete complex sentence.