Zero
Structure: if + present + present
Use: Always true 100%
e.g. If you heat water at 100ºC it boils.
First
Structure: if + present + future(will)
Use: Very realistic 90% + present or future.
e.g. If you study more you will pass the exam.
Second
Structure: if + past simple + would
Use: Hypothetical 0-2% present or future
e.g. If I were Chinese I would speak ChineseIf I won the lotto, I would buy Ferrari
Third
Structure: if + past perf. simple + would/have + Past participle
Use: a hypothetical situation in the past with effect in the past.
e.g. If I had known you were sick, I would have visited you.
Mixed
Structure: if + past perf. simple + would + verb
Use: a hypothetical situation in the past but effect is now.
e.g. If I had known she was depressed she would not be in the hospital now.