While is a subordinating conjunction. It is mainly used to indicate time. It can also show contrast.
Strike while the iron is hot.
Here the conjunction while shows time. More examples are given below.
Someone phoned while you were out.
While playing football, I sprained by ankle.
While can show contrast.
While some children learn fast, others need extra help.
While Peter is warm and friendly, his brother is quite moody.
Here the conjunction while shows contrast.
However can be used as a subordinating conjunction.
However poor he may be, he is not going to beg.
You can arrange the tables and chairs however you want.
However you look at this situation, it is quite challenging.
However can also be used as an adverb. In this case, it is followed by an adjective or another adverb.
However hard she tried, she couldn’t control her emotions.
Sometimes, however is used as a conjunctive adverb. In this case, it usually goes at the beginning of the sentence and is separated from the rest of the sentence by a comma.
Alan works three jobs in a day; however, he doesn’t earn much.
However can also go in other positions in a sentence.
He was confident of his success. His coach, however, wasn’t all that sure.
The word since can be used as an adverb and a conjunction. When it is used as a conjunction, it can express time and cause.
I haven’t met her since she moved to Vegas.
I have been waiting for him since morning.
Since he hadn’t bought a ticket, they refused to let him in.
Unless and until
Unless shows condition. It means ‘if not’. Until means ‘up to the time when’.
I won’t be able to open the box unless you give me the keys.
I will wait here until you come back.