It would be a real nightmare if there were no conjunctions to make sentences longer for more meaningful and attractive writing. For example; we'll be constrained to sentences like; I love running. I love playing football. I don't love doing chores. Etc.

What Are Conjunctions?
Simply put, conjunctions are used to join words, phrases or even clauses to form longer and more complex sentences. 
In the earlier examples, a simple conjunction can be used to combine all three statements;
I love running and playing football, but I don't love doing chores afterward.

Coordinating Conjunctions
These conjunctions are used to link words, phrases or clauses in a sentence. FANBOYS is a popularly used acronym for common conjunctions; for, and, nor, but, or, yet, and so.
I want to visit my high school friend, but I'm cash strapped, unfortunately. 
The meeting started on time, so all major decisions were reached before recess. 

Correlative Conjunctions
Correlative conjunctions are those that work in synergy when used in a sentence. Common examples include; neither/nor, either/or and not only/but also. e.g.
We are required to neither speak nor talk in an exam.

Subordinating Conjunctions
Dependent and independent clauses are joined by subordinating conjunctions. Such conjunctions can also signal a contrast, cause-and-effect relationship or any other sort of link between clauses. 
Subordinating conjunctions include since, because, as, though, although, whereas and while. Certain adverbs like until, before or after may also be used as conjunctions.
The kid usually goes out to play until it's close to his father's arrival. 
In the above example, until is an adverb which also acts like a coordinating conjunction joining the independent clause "The kid usually goes out to play" and the dependent clause "it's very close to his father's arrival."
In use, a subordinating conjunction is part of the dependent clause and is not necessarily confined to the middle of the sentence. The independent clause can also come after the dependent clause. 
Before your departure, ensure to return my textbook
It is also necessary to insert a comma if the independent clause comes after the dependent clause.
Thus, He ate my food because he was hungry or 
Because he was hungry, he ate my food can both be used

Starting a Sentence with a Conjunction
The erroneous misconception is not to use a conjunction when starting sentences. But this couldn't be further from the truth. As a matter of fact, a subordinating conjunction can be used to begin a sentence if the independent clause comes after the dependent clause.
It is also acceptable to use a coordinating conjunction at the beginning of sentences. When used, they often function to lay emphasis. But they should not be excessively used this way to prevent losing the force. You'll, therefore, want to use them scarcely. 
Go to class early. And make sure to keep your bag safe.
Take the kids to school. But don't forget to return early. 

Some Subordinating Conjunctions
There are so many different subordinating conjunctions to choose. Examples include before, provided that, although, supposing, as much as, as though, once, lest, only if, wherever, whenever, while, even though and even if. 

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