While

While is a subordinating conjunction. It usually introduces subordinating clauses of time. But sometimes while also expresses contrast.

Strike while the iron is hot. (Here while shows time.)
While you were sleeping I was working. (Here while expresses contrast.)

However

However can be used both as a coordinating conjunction and as a subordinating conjunction. As a coordinating conjunction however stands alone and is placed somewhere in the middle of the sentence.

All of my friends came for my birthday: John, however, didn’t turn up.

When however is used as a subordinating conjunction, it takes an adjective or adverb after it.

However poor he may be, he is not going to beg.

Since

As a conjunction since means ‘as’, ‘because’ and similar ideas.

Since he has apologized we will not take any actions against him.
Since it was raining we cancelled the match.

Unless

Unless means if not. It is used to express a condition. Note that it is wrong to use another not after unless.

Unless you give me the keys of the safe, you will be shot.
(If you do not give me the keys of the safe, you will be shot.)
Unless you work hard you will not pass.
(If you do not work hard you will not pass.)

Until

Until means ‘up to the time when’.

Will you please take care of my bag until I return?
We will wait until he comes.

Lest

Lest means ‘that … not’. It is therefore wrong to put another not in the following clause. Another point that needs to be noted is that the only auxiliary verb that can be used after lest is should.

Walk carefully lest you should fall.
Eat and sleep well lest you should fall ill.

As…as

This correlative conjunction shows how two people or things are equal in some way.

He is as tall as you are.
I earn as much as you do.

So…as

So…as shows contrast. It is used to show that two people or things are not equal.

He is not so old as he looks.
He is not so successful as his brother.

Than

Than is used after comparative adjectives and adverbs. Note that as and that are not used after comparatives.

His wife is taller than him. (NOT His wife is taller as him.)

Note that as is used in comparisons of equality. Than is not used in this way.

My wife is as tall as I am. (Used with positive adjectives)
He is stronger than you. (Used with comparative adjectives)
He is as strong as you are. (Used with positive adjectives)