Conditional sentences

1. How they are formed

Zero conditional: if + Present Simple, ... Present Simple
First conditional: if + Present Simple, ... will + V
Second conditional: if + Past Simple, ... would + V
Third conditional: if + Past Perfect, ... would have + V3
Mixed conditional: if + Past Perfect, ... would + V (or other mixed patterns)

2. When to use

Zero conditional: general truths and scientific facts.
First conditional: real possible future situations.
Second conditional: hypothetical/unlikely present or future situations.
Third conditional: unreal past situations (regret).
Mixed conditional: mixed time references in condition and result.

3. Examples

Zero conditional (general truth):

If you heat water to 100°C, it boils.

First conditional (real future possibility):

If it rains tomorrow, we will stay at home.

Second conditional (hypothetical):

If I had more free time, I would learn Spanish.

Third conditional (past regret):

If she had studied harder, she would have passed the exam.

4. Differences and notes

Zero vs First Conditional

Zero conditional states facts; first conditional talks about real possible future results.

Second vs Third Conditional

Second conditional is about unreal present/future; third conditional is about unreal past.

Mixed Conditional

Used when the condition and result refer to different times.