0 conditionals are conditionals which talk about something that is always true.

The structure is 'If + present + present'.

That is the verb  in the first clause is in the present tense and also in the second clause.

'If you mix blue and yellow, you get green.'

In the first clause (part of the sentence) the verb is 'mix' and in the second clause the verb is 'get'.

Here are some more examples.

'If you press the red button, the computer turns off.'

'If you  heat water to 100 degrees, it boils.'

We can replace 'if' with other words such as 'when' or 'whenever'

'Whenever I close the door, I hear a loud noise.'