There are two main types of clauses in the English language. They are known as the dependent clause and the independent clause. The dependent clause is a phrase that has a subject and verb in it, but this clause can’t stand alone.
This means that it can’t be a sentence because it most likely has a subordinating conjunction at the beginning of the clause. Subordinating conjunctions include when, while, because and after, to name a few. Then there are independent clauses that can stand alone as sentences.
They also contain a subject and verb, but they do not begin with a subordinating conjunction. In the example sentence of The pony express did not las long, this sentence is a independent clause because it can stand alone as a sentence.