Adjectives

These aren't very hard. They always end in {あ,い,う,お} + い. They never end in え + い which would instead be a noun. Basically you replace い with a form of か to inflect.

These inflections follow what a か + う verb would do. If you can conjugate 買う (かう, "to buy") you can conjugate every Japanese adjective. Just note that you don't say: "あつかせる" for "make something hot" you say "あつくする". For a command, say: "やさしくなって" ("be nice").

There is one peculiar formation of adjective which is almost dead in modern Japanese, but is visible in the phrase お早う御座います(おはよう ございます). In this phrase the adjective 早い (はやい, "early, fast") takes on the form 早う (はよう). This is an archaicism in modern Japanese preserved in this static phrase and in a few others such as お目出度う御座います (おめでとう ございます, "congratulations"). It's simplest just to memorize these few forms and not attempt to learn the process for other adjectives. But this form can be used occasionally for some pretty good jokes, like saying "おそよう御座います" to someone who's arrived late. It's from the adjective 遅い (おそい), meaning "late".