As stated in the very beginning section of this guide, all sentences have at least one subject and at least one verb, at minimum. For example, “Rain falls” is a sentence because it has both a subject (rain) and a verb (falls).
In North American academic English, this sentence is a simple sentence because the noun “rain” is the only subject in the sentence. However, some simple sentences will have more than one verb, as in:
Rain falls and splashes on the ground.
Likewise, some simple sentences can be even longer than this when all the all the words work together to form a complete idea about the single subject. For example:
Rain falls from the clouds that form from evaporated water warmed by the sun.
So a simple sentence isn’t a short sentence or a clear sentence. It is a sentence with a complete idea and at least one verb relating to just one subject.
For further examples as well as information on avoiding common mistakes with simple sentences, watch the video below.
A compound sentence combines two complete ideas with one of the following conjunctions: "for", "and", "nor", "but", "yet", "or", and "so".
For example, "Eagles fly, and penguins swim" is a compound sentence that consists of two complete ideas, each containing at least one subject ("eagles" and "penguins") and verb ("fly" and "swim"):
By contrast, the following sentences are not compound sentences, even though they each join two verbs with a conjunction ("and", in this case):
In the above examples, since the second part of the sentences are missing a subject, they cannot form a complete idea on their own. Think about it this way: the sentences cannot be divided into two simple sentences without adding another subject, so the sentence is not a compound sentence.
It may be helpful to compare the above examples with the following two sentences below:
Each of these two sentences are examples of compound sentences since they each have an additional subject. In other words, they each can be divided into two separate, simple sentences:
So in this sense, a compound sentence combines two complete ideas that could each form a simple sentence to create a sentence in which there are at least two things happening.
For more information on recognizing and punctuating compound sentences, please see the video below:
A complex sentence contains at least one clause that cannot stand alone as a simple sentence (it is not a complete idea). For example:
When the rain falls, I use an umbrella.
Although there is more than one subject and verb in this sentence, the only clause that can form a simple sentence is "I use an umbrella". When combined with the incomplete idea "When the rain falls", the two clauses work together to create a complex sentence.
Similar to "when", clauses starting with "if", "since", "because", "as", "although", "while", "until" , "after", and "before" do not express a complete idea and cannot stand alone as a simple sentence. Instead, these clauses provide information about time, place, cause or effect either at the beginning or at the end of a sentence, as in:
The sun rose before she woke up.
After they finished eating, they went home.
We cannot leave until he joins us.
Words like "before", "after", and "until" are sometimes called dependent markers, or subordinating conjunctions. When they start a sentence, they must be followed by a comma and a complete idea in edited academic English (as in this sentence).
For a complete list of subordinating conjunctions in dependent clauses, see The Subordinate Clause and for more information on how punctuate complex sentences, see Comma Usage in Compound and Complex Sentences.