A sentence fragment is a sentence that is missing either its subject or its main verb.

Unattached phrases must be made part of a complete sentence. You can join them either using a comma or a conjunction.

• INCORRECT: Since I was finished with my work, I got my phone out.  Texting my best friend about our plans later.
• CORRECT: Since I was finished with my work, I got my phone out, texting my best friend about our plans later.
• CORRECT: Since I was finished with my work; I got my phone out and was texting my best friend about our plans later.

Two clauses can be combined using a subordinating conjunction or an adverb to show the dependence of one clause on the other.

• INCORRECT: The children were running for the playground.  Because recess had just started. 
• CORRECT: The children were running for the playground because recess had just started. 

• INCORRECT: Since you were late to the movie you missed to the first half.
• CORRECT: Since you were late to the movie, you missed to the first half.