Conjugation of Japanese verbs is less complicated than Romance languages: There is only one conjugation per tense. The difference
between “you", “I" and “he/she" is based on the context of the sentence. Long-form is used in both writing and conversation, and is the middle level of politeness: long-form is more polite than short-form, but not as much as けいご (keigo), the most formal type of Japanese.
Ru-verbs get their name from the る (ru) at the end of the verb. When る (ru) is removed, you have the stem of the verb — the stem is used to conjugate. The example verb is たべる (taberu) “to eat":
Dictionary Form たべる (taberu)
Stem たべ (tabe)
Present, Affirmative たべます (tabemasu)
Present, Negative たべません (tabemasen)
ーます (masu) is the present affirmative verb ending and ーません (masen) is the present negative verb ending. These two verb endings are used in all long-form present conjugations.
The other main group of Japanese verbs, u-verbs, are slightly more complicated that ru-verbs. The suffix of u-verbs is u. For the verb いく (iku) “to go", the u is removed, leaving the stem ik. Like ru-verbs, the endings ーます (masu) and ーません (masen) are added, but with an i before each of the endings. To remember the i before the endings, think of the stem as いき (iki):
Dictionary Form いく (iku)
Stem いき (iki)
Present, Affirmative いきます (ikimasu)
Present, Negative いきません (ikimasen)
Some verbs that end in る (ru) can be u-verbs, such as かえる (kaeru) “to return":
Dictionary Form かえる (kaeru)
Stem かえり (kaeri)
Present, Affirmative かえります (kaerimasu)
Present, Negative かえりません (kaerimasen)
Before conjugation, it is important to know which type of verb you are using.
Two verbs that are commonly used in Japanese, する (suru) “to do" and くる (kuru) “to come" are both irregular verbs:
Dictionary Form する (suru)
Stem し (shi)
Present, Affirmative します (shimasu)
Present, Negative しません (shimasen)
Dictionary Form くる (kuru)
Stem き (ki)
Present, Affirmative きます (kimasu)
Present, Negative きません (kimasen)
Because irregular verbs do not follow the same patterns as ru-verbs and u-verbs, you should memorize the conjugation of these two irregular verbs.
Banno, Eri, Ohno, Yutaka, Sakane, Yoko and Shinagawa, Chikako. Genki I: An Integrated Course in Elementary Japanese. The Japan Times, 1999