They describe different probability/possibility: the higher the number, the less probable the situation.
Zero conditionals are always true in the right situation but aren't specific "if it rains, you get wet". This is true at any time.
Type 1 are the same, but specific: "if it rains, you'll get wet" This is about a specific incident and person.
Type two are unlikely or impossible: "if I won the lottery I'd go on holiday (I probably won't win so I won't go on holiday)", "if I were you, I'd study more (it's impossible for me to be you, but it's not impossible for you to study more).
Type three are past events with different outcomes:" if I had taken my car I wouldn't have been late (I didn't take any car and I was late) ", "if I had known the food was so terrible, I would have eaten in another restaurant (I didn't know and I ate there).
The further in the past the statement is, the more unlikely/impossible it is.
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