A compound sentence is made up of two independent clauses.
Lucy is tired. She will go to bed soon. 1 2
Both sentences are clear, but you want to join the sentences together to avoid using choppy sentences (we’ll come to those later). You can combine them either with a coordinating conjunction or with a semi-colon.
Lucy is tired; she will go to bed soon.
(That’s the one with the semi-colon. You need a semi-colon to join two independent clauses because the comma isn’t strong enough.)
And what is a coordinating conjunction?
Think FANBOYS – the following are all coordinating conjunctions
They all have slightly different meanings, which, as native speakers, we’re naturally aware of:
and = the 2nd clause contains the same type of idea
but = the 2nd clause contains an equal but opposite idea
or = the 2nd clause contains an equal choice
so = the 2nd clause contains an equally important outcome or result
If you’re joining two independent clauses together, the coordinating conjunctions always need a comma before them.
Lucy is tired, but she’ll go to bed soon.
Lucy is tired, so she’ll go to bed soon.
The ideas must always be equal for them to be next to each other and both clauses must make perfect sense on their own.