The simple sentence has one finite verb. It expresses a single main idea. The basic sentence has a subject and a verb. Example: I ran.
Sometimes for an imperative (or instruction) the subject is implied. Example: Be quiet. (“Be” is the verb in this imperative – and the subject is ‘you’, as you are the one who needs to do the verb – and be quiet!)
However, a simple sentence is not always very short. For example, in the idiom, Early to bed and early to rise, makes a person healthy and wealthy and wise, there is only one finite verb (“makes”) and one idea (it’s advisable to be an early sleeper and riser).