Final answer:
A simple sentence has one idea expressed in a single main clause, which can be short or long.
Step-by-step explanation:
A simple sentence has one idea expressed in a single main clause (also known as an independent clause). A main clause contains a subject and a predicate and can stand alone as a sentence. A simple sentence can be short or long, as shown in the examples below. The phrases in the long sentence add information, but the sentence remains a simple sentence nonetheless because it has only one clause.
The coronavirus spread around in the world in 2020.
School-age children and college students were pushed into virtual learning environments in March 2020, with schools closing for unspecified lengths of time.
Compound Sentences