Reading words containing the consonant digraphs sh (as in ship and crush), ch (as in chin and rich), ng (as in bring), th (as in this, think and Seth), ph (as in phonics), wh (as in which) and trigraph tch (as in catch).
Reading multi-syllable words.
There are small number of Heart words included in each book. These are words that children cannot decode (sound out) yet, so they need to learn any tricky bits off by heart.