Most British people and many international students do not know how to use apostrophes properly! Here’s a great picture to illustrate my point:
Of course, the sign is talking about lots of taxis, so it should be ‘TAXIS’. I was also recently looking through a dissertation that a student wrote with the title ‘The effect of globalisation on company’s profits’. Only one company? No, lots of companies! So it should be THE EFFECT OF GLOBALISATION ON COMPANIES’ PROFITS.
The trick is to work out whether what you’re thinking about is SINGULAR or PLURAL. Work out what you want to say WITHOUT THE APOSTROPHE by changing the sentence around. So the sentence above becomes:
THE EFFECT OF GLOBALISATION ON THE PROFITS OF COMPANIES
and I hope that you can see the starting word is COMPANIES. Then the rule is that for possession you add ’s. But if the word already ends in ’s’ you only add the apostrophe! Now what’s so hard about that??
Now read about whether you should write ITS or IT'S!
As usual I’ll leave you with a link. Here’s a handy page from the University of Teeside with an exercise which you can use to test yourself.