A conjunction may be used to indicate the relationship between the ideas expressed in a clause and the ideas expressed in the rest of a sentence.

1. Coordinate conjunctions

Coordinate conjunctions are used to join two similar grammatical constructions; for instance, two words, two phrases or two clauses.

and: in addition
    * She tried and succeeded.

but: however
    * They tried but did not succeed.

or: alternatively
    * Did you go out or stay at home?

nor: and neither
    * I did not see it, nor did they.

yet: however
    * The sun is warm, yet the air is cool.

2. Correlative conjunctions

Correlative conjunctions are used in pairs, in order to show the relationship between the ideas expressed in different parts of a sentence.

both ... and
    * He is both intelligent and good-natured.

either ... or
    * I will either go for a walk or read a book.

neither ... nor
    * He is neither rich nor famous.

hardly ... when
    * He had hardly begun to work, when he was interrupted.

if ... then
    * If that is true, then what happened is not surprising.

no sooner ... than
    * No sooner had I reached the corner, than the bus came.

not only ... but also
    * She is not only clever, but also hard-working.

rather ... than
    * I would rather go swimming than go to the library.

scarcely ... when
    * Scarcely had we left home, when it started to rain.

what with ... and
    * What with all her aunts, uncles and cousins, she has many relatives.

whether ... or
    * Have you decided whether you will come or not?

3. Subordinate conjunctions

Some Common Subordinating Conjunctions:
After, Although, As, As if, Because, Before, Even though, If, In order that, Since, So that, Though, Unless, Until, When, Whenever, Where, Whereas, Whether, while

As
    * because:  As he is my friend, I will help him.
    * when:  We watched as the plane took off.

After
    * later in time:  After the train left, we went home.

Although or though
    * in spite of the fact that:  Although it was after midnight, we did not feel tired.

Before
    * earlier than:  I arrived before the stores were open.

Because
    * for the reason that:  We had to wait, because we arrived early.

For
    * for, because:  He is happy, for he enjoys his work.

If
    * on condition that:  If she is here, we will see her.

Lest
    * for fear that:  I watched closely, lest he make a mistake.
Note the use of the Subjunctive Mood in the clause with lest.

Providing or provided
    * on condition that:  All will be well, providing you are careful.

Since
    * from a past time:  I have been here since the sun rose.
    * as, because:  Since you are here, you can help me.

So or so that
    * consequently:  It was raining, so we did not go out.
    * in order that:  I am saving money so I can buy a bicycle.
Note: When used with the meaning in order that, so is usually followed by that in formal English.
e.g. I am saving money so that I can buy a bicycle.

Supposing
    * if:  Supposing that happens, what will you do?

Than
    * used in comparisons:  He is taller than you are.

Unless
    * except when, if not:  Unless he helps us, we cannot succeed.

Until or till
    * up to the time when:  I will wait until I hear from you.

Whereas
    * because:  Whereas this is a public building, it is open to everyone.
    * on the other hand:  He is short, whereas you are tall.

Whether
    * if:  I do not know whether she was invited.

While
    * at the time when:  While it was snowing, we played cards.
    * on the other hand:  He is rich, while his friend is poor.
    * although:  While I am not an expert, I will do my best.

In addition, the following phrases are often used at the beginning of subordinate clauses.

As if
    * in a similar way:  She talks as if she knows everything.

As long as
    * if:  As long as we cooperate, we can finish the work easily.
    * while:  He has lived there as long as I have known him.

As soon as
    * immediately when:  Write to me as soon as you can.

As though
    * in a similar way:  It looks as though there will be a storm.

Even if
    * in spite of a possibility:  I am going out even if it rains.

In case
    * because of a possibility:  Take a sweater in case it gets cold.

Or else
    * otherwise:  Please be careful, or else you may have an accident.

So as to
    * in order to:  I hurried so as to be on time.