Form and function

Each word or unit belongs to a grammatical group. The word cat is part of the group of the nouns, that is what we call grammatical form. We can identify words by the group to which they belong as nouns, verbs, articles, adverbs and so on. Getting back to the word cat, such word can perform different functions within a sentence.

            [1] The cat is playing in the garden. (S)

[1a] I gave the cat a ball. (iO)

[1b] My pet is a cat. (sC)

[1c] I love my cat. (dO)

            In [1] the word cat has the function of subject (S), while in [1a] it is working as a indirect object (iO). Then in [1b] the same word performs the function of subject complement (sC), finally in [1c] it works as direct object (dO). This what we call grammatical function.