go for a walk, if the weather is fine.
If
he buys tickets, we will go to the theatre.
Условия маловероятны
Относится к настоящему времени:
I would go for a walk if the weather was fine. (But it is bad now)
If he bought tickets, we would go to the theatre.
Условия нереальны
Относится к прошлому:
I would have gone for a walk if the weather had been fine yesterday. (But it was bad)
If he had bought tickets, we would have gone to the theatre.
Drugs and Sport