I'm in favour of the idea so long as it's not too expensive. (PROVIDED / too much money)
I'm in favour of the idea, provided it does not cost / they do not spend... too much money
They won't achieve anything without popular support (UNLESS)
Unless they win/have/get popular support, they won't achieve anything.
If you've got a student card, you can get discounts. (PROVIDING)
You can get discounts providing you’ve got a student card.
If we do not send it express, it will take days (OTHERWISE)
We’d better send it express, otherwise it’ll take days.
The economy is still doing badly so it's easy for the opposition to attract new supporters. (LONG/ fails to improve)
As long as the economy fails to improve, the opposition will continue to attract new supporters.
Imagine if I don’t arrive till after midnight, will the guest-house still be open? (SUPPOSING)
Supposing I don’t arrive till after midnight, will the guest-house still be open?
What would Carlos do if he saw the girl from Student One again? (SUPPOSING)
Supposing Carlos saw the girl from Student One again, what would he do?
I know opinion polls aren't entirely reliable, but it doesn't look like they'll win an overall majority. (ASSUMING)
Assuming (the) opinion polls are reliable, it doesn't look like they'll win an overall majority.
If you don't make a mess you can play in the living room. (AS LONG AS)
You can play in the living room as long as you don’t make a mess.
It doesn't matter if you dn't like it. That doesn't stop it happening, does it! (WHETHER/ It's always going to happen)
It's always going to happen, whether you like it or not!
The problem with nuclear power is that if an accident happens, it could totally devastate the area. (SUPPOSING)
Supposing an accident happens/happened...Supposing there is/there was an accident, it would totally devastate the area.