A sentence is an independent clause. It can stand alone. It has a subject and a verb,
I am.
I eat pasta.

These independent clauses are simple sentences. They are very effective. However, if you speak or write using only simple sentences, it makes you sound like a small child. It is boring. It is monotonous. It is not the best English has to offer.

A dependent clause is not a whole sentence. It cannot stand alone.
How I know.
Which is crazy.

These are sentence fragments. However, if you join these dependent clauses, which we can think about as the baby part of the sentence, to an independent clause, which we can think about as the adult part of the sentence, the rhythm of what you want to say gets more interesting.

What happens if you have two ideas that you want to join into a longer, more Italian sounding sentence? This kind of sentence is called a compound sentence. It is a fine thing to do, as long as you follow the rules of grammar.

A compound sentence is made up of two independent clauses separated by a comma and joined together with a conjunction. The acronym FANBOYS will help you remember these special conjunctions- for, and, nor, but, or, yet, so.

I am crazy about you, and I want to move in with you.

I bet you didn’t know you had to put a comma in that sentence. Well, you do. If not, you are not writing in correct English.

A complex sentence is made up of an independent clause, plus one or more dependent clauses.

“I am crazy about you,” is an independent clause, but if you put the subordinator “although” in front of it, it will become dependent.

“Although” is a subordinator.
A subordinator is a word that makes a clause dependent.
Subordinators are words like “although,” “because,” “after,” “if,” “since,” “unless,” and “when” that when placed at the beginning of an independent clause have the power to subordinate it and make the clause dependent on another clause.

Although I am crazy about you. This dependent clause cannot stand alone. It is a sentence fragment.

However, we can make a complex sentence by joining the dependent clause with an independent clause. Of course, we will need to separate them with a comma.

Although I am crazy about you, I do not want you to move in with me.

Now, you have a beautiful complex sentence and an apartment to yourself.

If you mix simple, compound, and complex sentences you will be able to add personality to your writing and make it more musical and interesting.
You’re welcome.

It is very helpful to know the three types of sentences for any certification or English requirement test because it will help you find the right answers for reading and listening comprehension, as well as help you with punctuation for the grammar part of the test.