Group A: best, test, nest, rest
Here are some flash cards for you to use.
final blend st flash cards
Group B: helpful, harmful, careful, fearful, useful, watchful, painful, hopeful
Group C: thoughtful, colourful, doubtful, spiteful, delightful, wonderful, thankful, cheerful
Groups B and C may find this BBC Bitesize lesson on adding the suffixes -ly and -ful useful.
https://www.bbc.co.uk/bitesize/articles/zc2j96f