In Japanese, almost any type of word can be attached to the beginning of a noun to modify the meaning, or describe the qualities of that noun. However, depending on the type of word used, conjugation rules will differ slightly.
For
い-Adjectives, they are simply attached to the front of the noun in their standard (dictionary) form.
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早（はや）い車（くるま）。
A fast car.
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怖（こわ）い先生（せんせい）。
A scary teacher.
When identifying what an い-Adjective is, and what is not, remember that an い-Adjective’s い will always be outside of any kanji in the word.
面白（おもしろ）い - The い is outside the kanji, so it is definitely an い-Adjective.
綺麗（きれい） - The い is inside of the kanji, so it is definitely not an い-Adjective.
There are some, but very few exceptions.
嫌（きら）い - The い is outside of the kanji, but this is actually a な-Adjective.