What is the difference between a sentence and a sentence fragment?
In most cases, a sentence contains at least a subject and a verb and expresses a complete thought.
A sentence fragment is usually missing a subject, a verb, or both (e.g., “Wondering what to cook for dinner”).
In some cases, a clause might have both a subject and a verb, but also a subordinating conjunction. This makes it a dependent clause on its own, which is a sentence fragment (e.g., “After the second Act”).
A simple question to ask yourself to test if you have a sentence fragment is whether the clause contains a complete thought. If it does, then there is probably no problem.