Usage
The i form has many uses, typically as a prefix. These include:
To form polite verbs when followed by the -ます -masu ending: 行く iku → 行きますikimasu, 使う tsukau → 使います tsukaimasu.
To express a wish when followed by the ending たい tai: 食べたい tabetai: "I want to eat it", 行きたい ikitai: "I want to go". (The tai ending conjugates as an い i adjective.)
To express a strong negative intention when followed by -はしない -wa shinai: 行きはしないよあんな所 iki wa shinai yo, anna tokoro "no way I'm going someplace like that".
To form a command when followed by
-なさい -nasai: これを食べなさい kore o tabenasai: "eat this", あそこへ行きなさい asoko e ikinasai: "go over there".
-な -na: 真っすぐ帰りな massugu kaerina "go straight home": 仲良く遊びな nakayoku asobina "play nice". (Used with children, etc.)
To express that something is easy or hard when followed by -易い -yasui or -難い -nikui: したしみ易い shitashimiyasui: "easy to befriend": 分かり難い wakarinikui: "hard to understand".
To express excessiveness when followed by the verb -過ぎる -sugiru: 飲み過ぎる nomisugiru: "to drink too much". (sugiru can also be used with the stems of adjectives.)
To express doing something in conjunction with something else. When followed by the suffix -ながら -nagara, the verb becomes an adverb that means doing something while doing something else.
歩きながら本を読んだ arukinagara hon o yonda: "I read a book as I walked."
When followed by the verb -やがる -yagaru in yakuza speech, to express affronted contempt (a conjugation of opposite polarity to the honorifics) showing disrespect in the form of hatred combined with haughty/macho disdain for the doer/subject of the action/verb: 殺しやがる koroshiyagaru: "to have the <expletive> gall to kill <sbdy>" [e.g. --without my permission]. (The te form can be substituted for the i form.)
The i form also has some uses on its own, such as:
To express purpose, with に ni: 食べに行きました tabe ni ikimashita: "I went there to eat".
In formal honorifics such as お使い下さい o tsukai kudasai: "Please use this".
In conjunctions in formal writing.
For some verbs, the i form also forms part of related words in ways that are not governed by any general rules. For example:
The i form of 食べる taberu (to eat) can prefix 物 mono to form 食べ物 tabemono (food). Similarly with 飲む nomu (to drink) and 買う kau (to buy).
The i form of 賭ける kakeru (to bet) is a word on its own: 賭け kake, which means "a bet".
離す hanasu (to separate) can be suffixed to the i form of kiru (to cut) to form 切り離す kirihanasu (to cut off).