Sentence fragments are words that look like a sentence but are not. For a group of words to be a complete sentence, they must contain at least one independent clause. And if you are wondering what an independent clause is, it is any group of words that has both a verb and a subject. It can be independent.
For a sentence fragment to be corrected and made to become a complete sentence, it must be written again with the existing main clause of another sentence.
Incorrect: We decided. To have an adventure.
Correct: We decided to have an adventure.