The zero conditional usually describes universal truths that will always happen if there is a particular condition.
If water reaches 100C, it boils
If you put caesium in water, it explodes
You can, therefore, replace ‘if’ with ‘when’.
When water reaches 100C, it boils
When you put caesium in water, it explodes
Notice in the statements there is no ‘will’, ‘would’ or any other modal verb. This is because there is no chance the result won’t happen if the conditions do. Zero conditionals state that once something starts (the condition), then the result will be inevitable.
When you put food in the fish tank the fish eat it
If you sit in this sun for 2 hours I’m going to put sun cream on you
The zero conditional mostly uses the present simple (put, reaches, explodes, and boils), but can use the present continuous also, like the above.
Zero conditionals can also be open-ended and talk about things that will happen in the future.
If Spurs win tomorrow, they win the league
In the above, for example, Spurs will be playing tomorrow, which is in the future, and if they win the game, then they win the league.