Use the zero conditional for what's always true: If you heat ice, it melts. Use the first for one particular possible situation in the future: If you leave the ice cream in the sun, it'll melt.
Same situation, two choices. Pick an example.
If you heat ice, it melts.
One real path: every time the condition happens, the result follows.
If you leave the ice cream out, it'll melt.
One particular situation that may or may not happen.
| Rule | Zero conditional | First conditional |
|---|---|---|
| Form | if + Present Simple, Present Simple | if + Present Simple, will + base verb |
| Meaning | A general truth: every time | One particular future situation |
| Instead of if | when or whenever, same meaning | when only if you're sure it'll happen |
| Example | If you press this button, the light comes on. | If you press that button now, the alarm will go off. |
If you mix yellow and blue, you _____ green.
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No, it's correct too: it treats the situation as one future case. The zero conditional (it melts) presents it as a general truth.