Two very basic notions are form and function. Forms perform functions.

The girl loves bananas.

The form would be girl. The function would be loves.

The girls love bananas.

The form would be girls. The function would be love.

Grammar is often thought to be made up of two parts. Syntax and morphology

Definition: Syntax has to do with functions: it is about the way in which words join together to form sentences.

Definition: Morphology has to do with form: it is about the kinds of ending which the form of a word can take.

Example: love or loves