English clauses which are not imperatives must have a subject. Sometimes we need to use a ‘dummy’ or ‘empty’ or ‘artificial’ subject when there is no subject attached to the verb, and where the real subject is somewhere else in the clause. It and there are the two dummy subjects used in English:
It’s always interesting to find out about your family history.
[real subject]To find out about your family history is always interesting. (The real subject – the thing that is interesting – is ‘to find out about your family history’.)
There are five Dutch people in our village. (The real subject is the Dutch people – they are in the village.)
It as a dummy subject
We often use it as a dummy subject with adjectives and their complements:
It’s important to wear a helmet whenever you do any dangerous sport.
[real subject]Wearing a helmet when you do any dangerous sport is important.
Not: Is important to wear a helmet … (The real subject is ‘wearing a helmet when you do any dangerous sport’ – that is what is important.)
It’s useful to write down your passport number somewhere, in case you lose it.