A compound sentence is a sentence which has two independent clauses linked together. A compound sentence has to have the two clauses linked together by a conjunction of some sort. This may take the form of a correlative conjunction, a coordinating conjunction, or a semicolon functioning as a conjunction. A comma is often used to help offset the two independent clauses as well, although it is not usually needed grammatically.
One can contrast the compound sentence with both the simple sentence and the complex sentence. A simple sentence is a sentence in which there is a subject and a predicate, and in which a complete thought is expressed, allowing it to stand alone. For example, We run outside every day. is a simple sentence, as is The moon is white. A complex sentence, on the other hand, includes both an independent clause and a dependent clause. For example, the sentence, When stars fall, I like to make wishes. is a complex sentence, with stars fall the dependent clause, and I like to make wishes the independent clause.
To form a compound sentence, you take two independent clauses, which could serve as simple sentences by themselves, and link them with a conjunction. The most common type of conjunction used is the coordinating conjunction. There are seven coordinating conjunctions in English: for, and, nor, but, or, yet, and so. These seven can be easily remembered by the mnemonic FANBOYS, with each letter representing the first letter of each coordinator.
For example, we can take two simple sentences: Jane likes to watch football. and Bob learned to knit. We can then connect them with a coordinating conjunction to create a compound sentence like: Jane likes to watch football, and Bob learned to knit. or Jane likes to watch football, so Bob learned to knit. The coordinating conjunction we use determines the meaning of our compound sentence, and of course not all coordinators work for all independent clauses, but all independent clauses need to have at least one conjunction to be joined together.
A compound sentence can also use a pairing of words that help each other out, known as correlative conjunctions. There are four common pairings of correlative conjunctions: both and and, not only and but also, either and or, and neither and nor. For example, we can take the independent clauses: The moon is full. and The stars are out. We can then join them together using one of our pairings to get: Both the moon is full, and the stars are out. or Neither the moon is full, nor the stars are out.
A semicolon can also act as a conjunction to form a compound sentence. For example, we can take the two independent clauses we just used, and join them together with a semicolon to form: The moon is full; the stars are out. In this way we link the two clauses more closely than if we were to have them as fully independent simple sentences, but we don’t link them more explicitly than that.