We use the zero conditional when we want to talk about facts or things that are generally true. Scientific facts are often covered by the zero conditional: "When you heat ice, it melts."
The zero conditional uses if or when and must be followed by the simple present or imperative.
For example:
"When it rains, tennis lessons are held in the gym."
"If it rains, tennis lessons are held in the gym."
"When John is out of the office, I take his calls."
"If John is out of the office, I take his calls."
It is important to remember that this condition is not realated to the past, present or future - it only applies to things which are ALWAYS true.
Now read the following sentences and decide if they are zero conditional or not:
Link: How to use IF in conditional sentences