If we examine the words in any sentence, we observe that they have different tasks or duties to perform in the expression of thought.
Savage beasts roamed through the forest.
In this sentence, BEASTS and FOREST are the names of objects. ROAMED asserts action telling us what the beasts did. SAVAGE describes the beasts. THROUGH shows the relation in thought between forest and roamed. THE limits the meaning of forest showing that one particular forest is meant. Thus each of these words has its special office (or function) in the sentence.
In accordance with their use in the sentence, words are divided into eight classes called parts of speech….namely