definition
You use unless when...
It is used instead of if + not in conditional sentences of all types.
You can play in the living room _____ you don´t make a mess
as long as
are provided, / guarantees / accession could represent a risk to our collective safety. / Unless
Unless guarantees are provided, accession could represent a risk to our collective safety.
You use should instead of...
IF.
_____ I phone you, you can assume the train´s on time.
Unless
you can go out to play. / you finish / your homework, / As long as
As long as you finish your homework, you can go out to play.
you use had instead of...
IF in third conditional sentences
___ the Prime Minister were to resign, there would have to be a general election within 30 days.
If
Otherwise , / will be caught / our dear European Commission / with its pants down.
Otherwise , our dear European Commission will be caught with its pants down.
we talk about thing that might happen but which we think are unlikely.
___ I known you were waiting outside, I would have invited you to come in.
Had
the score is zero. / If / the shot is a miss,
If the shot is a miss, the score is zero.
We can use otherwise when...
we have conditional meanings
You´ve got to start studying, ______ you´ll fail all those exams.
otherwise
this structural weakness, / Given / the idea of a new Cold War is misleading.
Given this structural weakness, the idea of a new Cold War is misleading.