They are all conjunctions: - Since: because, as 'She won't be able to attend the wedding since her husband has recently died.' - As: used to state the reason for something 'She may need some help as she’s a foreigner.' We can also use 'as' (but not while) to express the idea that a change in one thing goes with change in another: 'As we drove further north, the weather got worse.' 'As she grew older she gained in confidence.' - So: we use it to express a result: 'We lost our way, so we were late.' 'I was tired, so I made mistakes.' - While: a conjunction meaning 'but': 'I'm right-handed while/whereas/but my brother is left-handed.' - Although: a conjunction meaning 'but': 'The team lost, though/although they played quite well.'