Conditionals are the "if-then" structures of English. They allow us to talk about possibilities, requirements, and regrets. There are four main types, and the secret to mastering them is matching the right tense to the right level of reality.
1. The Zero Conditional (Facts)
Used for things that are always true, like scientific facts or general habits.
Structure: If + Present Simple, ... Present Simple
Example: "If you heat ice, it melts."
Note: You can usually replace "if" with "when" without changing the meaning.
2. The First Conditional (Real Possibilities)
Used for specific future situations that are likely to happen.
Structure: If + Present Simple, ... Will + Verb
Example: "If it rains tomorrow, I will stay home."
Note: This is a "real" condition; there is a genuine chance it will occur.
3. The Second Conditional (Imaginary/Unlikely)
Used for hypothetical situations in the present or future. Things that are impossible or very unlikely.
Structure: If + Past Simple, ... Would + Verb
Example: "If I won the lottery, I would buy a boat."
The "Were" Rule: In formal English, use "were" for all subjects: "If I were you, I’d take the job."
4. The Third Conditional (Regrets/Past Hypotheses)
Used for situations in the past that didn't happen. It’s often used to express regret or relief.
Structure: If + Past Perfect, ... Would Have + Past Participle
Example: "If I had studied harder, I would have passed the exam."
Reality: I didn't study, and I didn't pass.
Quick Comparison Table

Type	Timeframe	Reality	Structure (If-Clause / Main Clause)
Zero	Any time	General Truth	Present Simple / Present Simple
1st	Future	High Probability	Present Simple / Will + Verb
2nd	Present/Future	Low Probability/Imaginary	Past Simple / Would + Verb
3rd	Past	Impossible (Already happened)	Past Perfect / Would Have + Past Participle

Important Tips
The Comma: If the "if" clause comes first, use a comma. If the "if" comes in the middle, no comma is needed ("I'll go if you go").
No "Will" in the If-Clause: We almost never put "will" or "would" inside the "if" part of the sentence.
Incorrect: If I will see him...
Correct: If I see him...