Conjunctions are words that link two parts of a sentence. Different kinds of conjunctions join different kinds of grammatical structures. There are three kinds of conjunctions: coordinating conjunctions, correlative conjunctions and subordinating conjunctions.
A coordinating conjunction joins two independent words, phrases or clauses, which are grammatically similar. For example: We can draw lessons from the past, but we cannot live in it - Lyndon B. Johnson.
Correlative conjunctions are formed by the combination of some conjunctions, which means that these are always used in pairs. This shows the relationship between ideas expressed in different parts of speech. For example: Polonius said, “Neither a borrower nor a lender be.”
Subordinating conjunctions join a dependent clause and an independent clause, where one clause is dependent (or ‘subordinate’) upon the other. These conjunctions are mainly placed at the front of the clause. For example: Some people make headlines while others make history - Philip Elmer-DeWitt.
A subordinating clause appears at the beginning of a subordinate (or dependent) clause, and a comma is needed at the end of the adverbial phrase when it precedes the main (or independent) clause. Example: Unless we act now, all is lost.