For a start - spelling and pronunciation! Look at: through though, thought, thorough, through, trough, tough, bough. All look similar, all sound very different.
And then there is the small matter of all the different form used in different parts of the world-wide English-speaking world. E.g. In Britain, pedestrians walk on the pavement ("sidewalk" in the USA), but in the USA cars and trucks drive on the pavement. And American biscuits are different from British biscuits ("cookies" in the USA). Etc, etc. In Australia "crook" is an adjective meaning misshapen or ill, but in Britain "a crook" is a criminal. And in Australia "rooted" is a very naughty word.