English Conjunction

Conjunctions are grammatical connectors that link words, phrases 
or clauses. A conjunction can indicate the relationship between 
the elements that it connects in the sentence. There are three 
types of conjunctions: coordinating, correlative, and 
subordinating.   

Coordinating Conjunctions

A coordinating conjunction connects words, phrases, and clauses 
that have equal or the same grammatical functions, The 
coordinating conjunctions include 'and', 'but', 'or', 'yet', 
'nor', 'for', 'and so'. For examples:

Connecting nouns: Mary will buy a computer and a scanner.
Connecting verbs: Jack did not call nor write his teacher.
Connecting adjectives: The toy was big but weird.
Connecting dependent clauses: If the students show and the teacher 
is prepared, class will be productive.
Connecting independent clauses: Two thousand students applied to 
the university, but only five hundred were admitted.   

Correlative Conjunctions

A correlative conjunction is a coordinating conjunction that works 
in pairs to connect elements in a sentence. The correlative 
conjunctions include 'both...and', 'not...but', 'not only...but 
also', 'either...or', 'neither...nor', 'although...yet', and 
'whether...or'. For examples:

Connecting nouns: The name of the computer is not Dall but Dell.
Connecting adjectives: Your employer should provide both health and 
life insurance.
Connecting prepositional phrases: Apple juice is made either by 
squeezing apples or by mixing a can of frozen concentrate.
Connecting independent clauses: Not only did Jack eat the rice, but 
he also ate the bread.   

Subordinating Conjunctions

A subordinating conjunction connects elements with different 
grammatical functions, usually a dependent and an independent 
clause. The subordinating conjunctions include 'after', 'in case', 
'unless', 'although', 'in that', 'until', 'as', 'now that', 'when', 
'as if', 'once', 'whenever', 'as though', 'since', 'where', 
'because', 'so', 'whereas', 'before', 'so that', 'whether', 'even 
though', 'than', 'which', 'except that', 'that', 'while', 'however', 
'though', 'who/whom', and 'if'. For examples:

Joe acts as though he has done something wrong.
Jack is not sure that the teacher will give him the good mark.
When the bell rings, the students must keep quiet.
Since the dog disappeared, the boy has been sad.

Conjunctive Adverbs  

A conjunctive adverb is an adverb that connects independent 
clauses. Some of the most common conjunctive adverbs are 
'however', 'moreover', 'nevertheless', and 'therefore'.   

Usage

Conjunctive adverbs require semicolons:
The project will probably be successful; however, I don't know 
when it will be.

Common Pitfalls

Conjunctive adverbs are often confused with coordinating 
conjunctions such as 'and', 'but', 'for', 'nor', 'or', 'yet' 
or 'while'. One difference is that coordinating conjunctions 
join clauses of equal rank and conjunctive adverbs do not. 
Another difference is that conjunctive adverbs are not true 
linking devices themselves, as indicated by their needing 
semicolons:

I don't have enough butter for my bread; therefore, I'll buy 
butter to make my bread taste better.

Flexibility 

The flexibility of the conjunctive adverb in the sentence also 
indicates they aren't true linking devices like coordinating 
conjunctions. For example, the second part of this sentence 
could also be written:

...I will therefore buy some butter to make my bread taste better.