i-adjectives are one of the two main types of adjectives in Japanese. They are called 'i-adjectives' because their dictionary form always ends with the sound 'i' (い). Unlike na-adjectives, they can be used directly before a noun without any extra particle, like おおきい いぬ (a big dog). They also conjugate like verbs to show tense and negation.

To make a negative statement, you change the final 'i' to 'ku' and add 'nai'. For example, たかい (expensive) becomes たかくない (not expensive). For the past tense, you change the final 'i' to 'katta' for affirmative, and 'ku nakatta' for negative. For instance, たかかった (was expensive) and たかくなかった (was not expensive).

When using an i-adjective to modify a noun, you simply place it before the noun. When the adjective is the predicate of a sentence, you can add the copula 'desu' for politeness, but the adjective itself changes form for tense and negation, not 'desu'.