As Sky News presenter Dermot Murnaghan knows all too well, the president of Estonia does not like it when you get things wrong.
One Donald Trump supporter found that out the hard way this week as Toomas Ilves publicly shamed him for making a grammatical error.
The Trump fan tried to criticise the president for tweeting in English - but, crucially, used "your" instead of "you're". Big mistake.
Ilves pointed out his error, explaining that not many people understand Estonian, before ending with the Latin "quod erat demonstrandum (that which was to be proved)" - which is often used in mathematics when something has been proven or explained.