Japanese

Conjugating verbs to the negative present tense in the plain form varies depending on the verb's ending. There are three types of endings we need to pay attention to:

1. Group 2, or 一いち段だん verbs, which all end in -る
2. Group 1, or 五ご段だん verbs that end in -う
3. Group 1, or 五ご段だん verbs that do not end in -う
4. The irregular verbs です、ある、する、来る

For 一いち段だん verbs, replace the -る with -ない. For example, 食べる becomes 食べない.

I do not eat cheese because I have an allergy.

アレルギー　が　ある　から、　チーズ　を　食たべない。 arerugii ga aru kara, tiizu wo tabenai.

For 五ご段だん verbs that end in -う, change the -う into -わない. For example, 買う becomes 買わない.

Though her voice is beautiful, she doesn't sing.

彼女かのじょ　の　声こえ　は　美うつくしい　けど、　歌うたわない。 kanozyo no koe ha utukusii kedo, utawanai.

For 五ご段だん verbs not ending in -う, change the う-sound into an -あ-sound, and add ない. For example, 行いく becomes 行いかない.

I won't go again.	もう　行いかない。 mou ikanai.
Although (I) don't understand it, (I) like it!	分わからない　けど、　好すき　だ　よ！ wakaranai kedo, suki da yo!

And finally, the negative plain form for the irregular verbs is as follows:

Plain Form Positive	Plain Form Negative
だ	じゃない
ある	ない
する	しない
来る	来こない

And here are example sentences for each of them:

It is not the real thing.	本ほん物もの　じゃ　ない。 honmono zya nai.
Don't you have a good idea?	いい　アイディア　が　ない　か？ ii aidhia ga nai ka?
(I) won't do it.	しない。 sinai.
I won't come today.	今きょ日う　は　来こない。 kyou ha konai.