Why it's important to teach through these groups??
If you will look at the first group it has s,a,t,i,p,n now wat do you think how many words will you be able to make out ot these sounds to read? The answer is sooooo may. For ex at, in, sit, nap , pant, pan, tap , sap and the list goes on... So you don't have to wait for your child to learn A to Z sounds first then blending. You can teach the first 6 sounds then teach the words with these sounds and then move to the next group. So the child can learn the blending simultaneously.
Now think if you are teaching A to z in sequence how many words you will be able to teach with A, b, c ,d ,e ??? Hardly any. That's why the these groups has been designed . I hope it will be helpful for you all.
