All i adjectives end in ai, ii, ui or oi. There
are none ending in ei. Not all words ending in i are i
adjectives. Kirai, "dislike", for example, is a na
adjective, so it forms a positive as kirai da and negative as
kirai ja nai. Some words can be both i adjectives and
na adjectives, such as ōkii and ōki na.
Many i adjectives end in shii, particularly adjectives related to
human emotions such as tanoshii (楽しい), "enjoyable",
kanashii (悲しい), "sad", or muzukashii (難しい),
"difficult".
Confusion may be caused when an i adjective takes the sou
ending: see 2.2.1. What are the uses of the sou ending?.
Acknowledgements
This entry was partly created by Paul Blay when it was
part of the "wiki sci.lang.japan FAQ" (see
17.3. FAQ Format).