Edward D. Johnson on sentence construction:
“Write in whole sentences, not in fragments….The fragment is easy to see in I discovered the overalls. When I was ladling out the chowder. The second “sentence” is merely a dependent clause of the first sentence and shouldn’t be separated from it by a period; in this example even a separating comma would be wrong (the when clause is a defining construction; see Rule 2-1)….
Such fragments are surprisingly common…after all, it’s hard to see what isn’t there…
…and it’s what isn’t there that makes a sentence or clause a fragment. Whenever something seems wrong with a sentence but it’s not clear what, check for fragments.”
Johnson’s book, The Handbook of Good English, is an excellent resource for writers of all kinds. You can find it here.