EXPLANATION To add information about verbs (to modify them) we can add adverbs, adverbial phrases and adverbial clauses. This is how we might modify the verb 'skidded'.
Adding an adverb:
Adding an adverbial phrase:
Adding an adverbial clause (i.e. it has its own verb):
In the last example, a clause has been added. That adverbial clause is part of the longer sentence and so it should not be punctuated as a separate sentence.
Adverbial clauses can be introduced by many different conjunctions, such as because, as, when, whenever, although, if and where. Just as arms cannot exist without a body, an adverbial clause cannot stand alone: it is a dependent subclause within a sentence; that is, it is a subordinate clause - and it is joined to the main sentence by a conjunction.
If an adverbial clause has been separated from a sentence, we say that the sentence has been fragmented. That is what has happened in the typical error above. The adverbial clause (though to do that they sometimes have to reduce individual freedoms) has been punctuated as a separate sentence, a fragment, instead of being included in the sentence.
Adverbial clauses are often marked off by a comma. Hence, the problem of fragmentation is possibly partly caused when people hear that pause in their mind and insert a full stop (instead of a comma).