What if's

Hello Jack

1. What if there is a fire, what would you do? (I hear a lot of people that use 'is' with 'would', is that correct? If it is correct, is it a mixed conditional?)

Yes, mixed: somewhere between —

a) What if there's a fire - what'll you do? (immediate: you want to convey the fact that you think it's a real possibility.)

b) What if there were a fire - what would you do? (remote: you want to discuss the possibility of the fire, but you don't want to make it sound too immediate - perhaps you don't want to frighten the other person; perhaps you really don't believe there's much likelihood of a fire).

So your mixed version has an odd effect: in the IF clause, the speaker presents the fire as an immediate possibility; in the main clause, he treats it as remote. (Perhaps he was talking to an old lady, and saw the look of alarm on her face as he said 'what if there's a fire?'. So he changed to 'would' - to make it less frightening...)

2. What if there is a fire, what will you do?
— as above.

3. What if there is a fire, what are you going to do?
— this makes the fire seem even more immediate. 'What are you going to do?' is what you say when something really does have to be done.

4. What if there was a fire, what would you do? (So this is conditional #3?)
This is a version of a), above: 'what if there were a fire...' So it's a type 2 conditional.

'Was' is often used by speakers instead of the subjunctive 'were', esp. in BrE.

MrP