Use Zero Conditional to talk about GENERAL TRUTHS, FACTS, or things that ALWAYS happen. Both clauses use PRESENT SIMPLE. Structure: IF + present simple, present simple.
Quick tip: IF + present simple, present simple = general truth that's ALWAYS true. Use for facts, science, universal rules!
Form: IF + present simple, present simple
Both IF clause and result use present simple. No future, no past. For general truths that are always true.
If you heat water to 100°C, it boils. (always true - fact)
If it rains, the ground gets wet. (always happens)
If I don't sleep, I feel tired. (general truth about me)
Plants die if they don't get water. (scientific fact)
Use: General truths and facts
Use for scientific facts, general rules, things that always happen, or universal truths.
If you mix red and blue, you get purple. (always true)
If you press this button, the light turns on. (always happens)
If I eat too much, I feel sick. (general truth)
Water freezes if the temperature is below 0°C. (scientific fact)
When vs IF
You can use WHEN instead of IF in Zero Conditional - both mean the same for general truths.
If/When you heat ice, it melts. (both correct)
If/When I'm tired, I go to bed. (both correct for general truth)
Common mistakes
Not "If you heat water, it will boil." — say "If you heat water, it boils.". Zero Conditional uses present simple in BOTH clauses (not will). It's a general truth, not a future possibility..
Not "If it rains, it rained." — say "If it rains, the ground gets wet.". Both clauses use present simple for general truths. Don't mix tenses in Zero Conditional..
Practise it
Is it a FACT that's ALWAYS true? Use Zero Conditional! IF + present simple, present simple.