While is usually a subordinating conjunction of time. It can also be used to show contrast.
- Strike while the iron is hot. (Subordinating conjunction of time)
- Thieves broke into the house while they were sleeping. (Subordinating conjunction of time)
- While she is a doctor I am a sweeper. (Subordinating conjunction showing contrast)
- While he leads a luxurious life, his old parents are struggling to make both ends meet.
However can be used in two ways. When it is a coordinating conjunction it can stand alone and is generally placed somewhere in the middle of the sentence.
- He is very intelligent. However, he lacks experience.
- The whole class was present. He, however, remained absent.
When however is used as a subordinating conjunction, it takes some adjective or adverb after it.
- However poor he may be, he is not going to beg.
Unless, until and lest
Unless means if not. It shows condition.
- I cannot open the door unless you give me the key. (= If you do not give me the keys, I cannot open the door.)
Until and till can be used both as prepositions and conjunctions. They mean exactly the same.
- Will you please wait here until/till I return?
- I will wait until/till I hear from you.
Lest also shows condition.
- Take exercise regularly lest you should fall ill. (= Take exercise regularly so that you will not fall ill.)
As long as
As long as can refer to a period of time.
- We will have to stay indoors as long as the rain continues.
As long as can also be used to state conditions.
- You can take my camera as long as you use it carefully.