
Max M. answered 11/14/18
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A complete sentence needs two things: something happening, and someone or something doing that thing. The fancy terms for these are "predicate verb" and "subject," but that's less important than understanding that every sentence has something happening in it. That something that happens could be an action like run or swim, or something that doesn't look like an action, like think or wish, or even be.
The tricky part is it has to actually be happening in the sentence. Some words, like "because," or "which," tell you that the part that follows isn't actually happening, but is just added information to what came before.
So when I say, "I ate dinner quickly," that's a complete sentence because something is happening (eating dinner), and someone is doing it (me). However, if I say, "because I like pizza," that's not a sentence, because even though "like" is an action, and I am doing it, "because" tells you that it's not happening now; it's just information you need to understand why something else is true. And if that something else isn't in the sentence, then it's not a sentence; it's a fragment.
So:
I ate dinner quickly. (Complete sentence)
Because I like pizza. (Fragment)
I ate dinner quickly, because I like pizza. (Complete sentence)
Does that help?