Tricky words are taught along side the letters and digraphs from phase 2 on wards . These words can't be sounded out for reading and writing, children need to just remember the spelling of these word. A lot of these words go on to be part of the Year 1 and 2 'Common Exception Words'.
Set 1 tricky words begin within phase 2 in reception and continue to be taught throughout the phases. Some children pick them up quickly and other find it a little trickier. Children can make good progress phonetically but the tricky words just don't click, they will you just need to support regular practise at home.