Avoid sentence fragments.
When you begin a sentence with a capital letter, and conclude with an end mark, but forget to insert a main clause, you have written a sentence fragment. This is a fierce error in writing. Read these examples:
Because hungry whales stormed the beach.
Running all over his new white sneakers.
To buy ice cream for his needy friend.
There is no main clause in any of the above and therefore you have sentence fragments.
Read the revisions below. By adding a main clause we complete the thought:
Because hungry whales stormed the beach, Sue and Jane decided to call animal control.
Leo grabbed the tea kettle too quickly, spilling boiling, hot water all over his new white sneakers.
Harry used his paper route money to buy ice cream for his needy friend.