"What's the difference between four and since? That's a question that students ask all the time," he says. "When you say four, when you're about to describe the length of time that you have spent doing something, since sets the date that you started." 'If I had bought a lottery ticket, I would have won the lottery,' another student asks. "'I've been doing something for 10 years' - what's the difference?" an English teacher is asked by his native speaker. ''And if I bought a Lottery ticket, I'd win the lottery.' And so it goes on until we get through this one,'" Mr Rennie concludes.