FANBOYS is an acronym for the seven words recognized in English as coordinating conjunctions. Those words are
Using these words is an acceptable way to join two or more nouns, verbs, and many other grammatical constructions, including independent clauses. When independent clauses are joined they form a compound sentence.
The FANBOYS acronym is an easy way for children to remember which words can be used to form compound sentences. If one of these coordinating conjunctions is used, then a comma must be used after the first clause unless the clause has only a few words.
Some people use “then” as if it were a coordinating conjunction, but it isn’t. “Then” is an adverb and cannot join two clauses unless a coordinating conjunction is also used.
Another way to form a compound sentence is to use a semicolon. When a semicolon is used, no coordinating conjunction is used. Clauses joined by a semicolon must be related in content.
Other conjunctions, called subordinate conjunctions, are used to join one independent clause and one or more dependent / subordinate clauses. Complex sentences join two clauses of unequal weight while compound sentences usually join two clauses of equal weight.
Beginning readers needn’t know about coordinating conjunctions. By third grade students are learning rules of grammar. That is when they usually encounter FANBOYS for the first time.
Beginning writers sometimes think that if they use a coordinating conjunction to join two little sentences, they are writing better. Sometimes they are. But sometimes they are just creating stringy sentences.