Should is often used in conditional clauses expressing possibilities, suppositions etc. By using should in the if-clause we are suggesting that something is unlikely or not particularly probable.
Example: Should your child become anxious or nervous about any activity, it is a good idea to inform the team-leader. (or If your child should become …)
Unless. Conditional clauses can begin with unless. Unless means something similar to ‘if … not’ or ‘except if’.
Example: Unless I phone you, you can assume the train’s on time. (If I do not phone you /except if I phone you, you can assume the train is on time.)
If + were to. In formal situations, we can use if + were to when we talk about things that might happen but which we think are unlikely.
Example: Were we to give up the fight now, it would mean the end of democracy in our country. (If we gave up the fight now …)
As long as. Sometimes we need to impose specific conditions or set limits on a situation. In these cases, conditional clauses can begin with phrases such as as long as, so long as, only if, on condition that, providing (that),provided (that).
Had you (Had with inversion) - In formal situations, we can use had + subject + verb instead of if in third conditional sentences.
Example: Had I known you were waiting outside, I would have invited you to come in.(If I had known you were waiting outside …)