A simple sentence is one that contains only one, single independent clause.
In simpler terms, it can be understood as a sentence that is
Grammatically complete
Has only one idea in it
e.g. It has leathery skin. (One idea, grammatically complete)
A compound sentence is one that has two independent clauses (simple sentences) joined by a coordinating conjunction (a FANBOYS), e.g.
It has razor-sharp teeth and I have very soft skin.
My advice to you is to buy a harpoon for you’ll never kill it otherwise.
I am not hopeful nor do I see any point in running away again.*
A complex sentence is one that contains one independent clause and a dependent clause. The dependent clause will start with a subordinate conjunction.
The order of the clauses is flexible: either the independent clause or the dependent clause can come first, e.g.
Independent clause first, then subordinating conjunction starting the dependent/subordinate clause that comes last, e.g.
I was swiftly gobbled up even though I was wearing crocodile repellent.
Dependent/subordinate clause first (with subordinating conjunction at the start), then comma before the independent clause e.g.
Even though I was wearing crocodile repellent, I was swiftly gobbled up.
A compound-complex sentence is one that contains at least two independent clauses and a dependent clause.
The two independent clauses give it the ‘compound’ part, and the independent + dependent combination gives it the ‘complex’ part.
I want to escape the crocodile, but my friend has given up because it’s too hard.
I want to escape the crocodile = independent clause
but my friend has given up = independent clause with coordinating conjunction at start
because it’s too hard = dependent/subordinate clause with subordinating conjunction at start
Remember: a dependent clause will always start with a subordinating conjunction; a subordinating conjunction will always start a dependent clause.
Tip: to work out if a sentence is a compound, complex, or compound-complex sentence, look for the conjunctions:
If there are only coordinating conjunctions in the sentence (and at least one is being used to connect two independent clauses), it’s a compound sentence.
If there are only subordinating conjunctions in the sentence (which will, by definition, in every case be connecting a dependent clause to an independent clause), it’s a complex sentence.
If there’s a coordinating conjunction and a subordinating conjunction in the sentence, it’s a compound-complex sentence.
When a sentence is intentionally written to not be grammatically complete, it is called a minor sentence, e.g.
Eating my leg.
Here, the writer intentionally wrote this minor sentence. This was the response that Character A gave after Character B asked “What did I hear the crocodile doing through the night?” The writer chose to have Character A speak in this minor sentence because it mimics how we often speak in conversation; this gives it a (sort of silly) sense of realism. Also, it helps to convey Character A’s frustration or pain: they cannot form complete thoughts because of the frustration or pain of the situation.
This is similar to a sentence fragment, but a minor sentence is used to talk about a sentence that is incomplete for effect rather than due to an error.