Because the short-term parking lot was full, Miranda had to use the park-and-ride lot and almost missed her plane.

The short-term parking lot was full, Miranda had to use the park-and-ride lot and almost missed her plane.

The second sentence is a run-on because two independent clauses are joined with only a comma. The first sentence corrects the error by making the first independent clause subordinate with the subordinating conjunction Because.

For more help, see section 20.