In Estonian grammar, adjectives play an important role in describing and modifying nouns. Adjectives can be used to express various qualities such as size, color, shape, and more.

One key aspect of adjectives in Estonian is that they agree with the noun they modify in case, number, and definiteness. This means that the ending of the adjective changes depending on the case and number of the noun. For example, if the noun is in the accusative case, the adjective will also have an accusative ending.

Another important concept in Estonian adjective theory is the distinction between short and long adjectives. Short adjectives are used to describe indefinite and general characteristics, while long adjectives are used to describe definite and specific characteristics. The distinction between short and long adjectives affects the declension patterns and the agreement with nouns.

In addition, Estonian adjectives can also be compared to express degrees of comparison, such as the comparative and superlative forms. These forms are created by adding specific endings to the adjective base.

Overall, understanding the theory of adjectives is crucial in mastering the Estonian language and expressing oneself accurately and fluently.