Who said hello?
Louis Armstrong said "Hello!" to Dolly Levi.
In fact he sang a whole song about it.
They wrote a play and then made a movie about it.
If you merely say just the one word "Hello" to your friend you are not even saying a sentence you are only using one word. If you said "Hello, Dolly, how are you? You are using a simplex sentence. If you said "Hello, Doll, I have not seen you in a long time and I was wondering how you were doing." That would be a compound sentence.