A simple sentence expresses one idea. The black horse galloped smoothly to the barn with her colt. Deaf people use their eyes very well, so they do not need help driving.
Most people use their ears for hearing the car motor, but deaf people feel vibrations.
Usually this is done only when the first idea is general, and the second illustrates the first or when the two ideas are clearly related. When a writer connects two or more ideas with a certain kind of connecting word, that makes a complex sentence. If the connecting word is between two ideas, no punctuation is used. If the connecting word begins the sentence, put a comma at the end of the first idea. I will meet you for lunch unless you call to cancel.