Here's why:
* Prefixes are added to the beginning of a word to change its meaning (e.g., "un-" in "unhappy").
* Suffixes are added to the end of a word to change its meaning or grammatical function (e.g., "-ing" in "running").
The words in the sentence "Thomas was a cheerful lad" are all base words, meaning they don't have any prefixes or suffixes attached to them.