A closer look
Try the substitution test. Swap in a noun like dog, verb like run, or adjective like big where the word stands. The match names its category; a word fitting none, like ouch, belongs to a smaller class.
Some words cross categories. Work is a noun in hard work, a verb in they work; test it by the slot it fills in context.
Relevant types and examples
Words change category by position. Work is a noun in hard work, where it follows an adjective, and a verb in they work, where it takes -s with a subject. When a word seems to fit two slots, use the whole sentence to decide which slot it fills, and classify it by that role in context.