When is a Sentence, Not a Sentence?

When is a Sentence, Not a Sentence?
A sentence always begin with a capital letter and end in either a full stop, exclamation or question mark.

I am UKgoodbye’ blog writer.
Complex sentences are sentences that contain an independent clause and secondary dependent clause:
The dog barks because it is hungry, but the cat meows because it is tired, or when it wants to be stroked.