"The cow washed it's clothes yesterday" was wrong, saying I should've put "its" instead. Is this a bug or am I this terrible at english despite it being my first language??
A very common error, even amongst native speakers! 'Its' only has an apostrophe when it's a contraction of 'it is'. If you remember that you don't have an apostrophe in 'his' or 'hers', you will get it right!