The vowel sound heard in cat is the short /a/ vowel sound. The most common spelling is a. There are, however, other spellings of the short /a/ vowel sound. These include ai in plaid, au in laugh and aunt, al in calf and half, and a-e in have.
Please note that the decodable book Nan and Pap should be used after lesson 3 when students have all the necessary tools to read it.