zero conditional vs first conditional

[Unregistered]

I can´t notice the difference between them.

[Horsa]

The zero conditional is used for universal truths e.g.

'If you heat water to 100C at sea level, it boils.'

It can also be used when the speaker believes that when the condition is fulfilled the result is always the same e.g.

'If I am even 5 minutes late for work, my boss shouts at me.'

It is important to understand that the 'universal' truth of the last example is entirely in the mind of the speaker and that it doesn't allow for the situation when the boss might be absent or late too.

The first conditional is used when we want to say that when the condition, which must be quite possible, is fulfilled the result, in the speaker's opinion, should be or is very likely to be that stated. E.g.

'If you shout at your boss, he'll fire you."

So, the zero conditional is for stating what the speaker regards as fact whereas the first conditional is for predicting the result of an action or particular situation.

However, whether something is fact or prediction is left for the speaker to decide. This means that some people may regard a particular result to be a fact while others may decide it is a prediction and each will choose to use a different conditional. This is why you are confused on the matter.