As a conjunction than follows an adjective or adverb in the comparative degree.
- Wisdom is better than riches.
- He is wiser than I am.
- I am smarter than you are.
Lest is used as a subordinating conjunction expressing a negative purpose. It has a similar meaning to ‘for fear that’.
- He fled lest he should be killed.
Note that lest is rare in modern English. Also note that the only auxiliary that can follow lest is should.
While is used to mean:
a) during the time that; as long as; as
- While there is life, there is hope.
- While they were sleeping, the robbers broke in.
b) at the same time that
- The boys sang while the girls danced.
- While you were playing I was working.
- While I have no money to spend, you have nothing to spend on.
As a conjunction only means ‘except that’ or ‘but’.
- The book would be helpful to you, only it is expensive. (= The book would be helpful to you, but it is expensive.)
Because, for and since
All of these words can be used to refer to the reason for something. Since clauses often come at the beginning of sentences.
- Since he had not paid his bill, his electricity was cut off.
A because-clause is less formal than a since-clause.
- Because I was ill, I could not attend the meeting.
- Since I was ill, I could not attend the meeting.