What is an irregular verb?
In all languages, verbs can be grouped into categories, based on how they conjugate. Typically, you will have a small number of regular groups, which have the same conjugation patterns, and then one irregular group, made up of verbs that do not fit into the others. Sometimes these can be grouped together, and sometimes they have their own individual rules. Japanese has three verb groups, the -ru verbs, such as miru (to see) or taberu (to eat), the -u verbs, such as au (to meet) or kaku (to write), and the irregular verbs. Luckily for all students of Japanese, there are only two irregular verbs, making this last group great for a quick study. The two verbs are kuru (to come) and suru (to do), and as they are very common, you may have already encountered some form of them in your studies.
Irregular Verb: kuru
This Japanese verb, meaning "to come", is written as kuru in its dictionary form. The present tense conjugation of this verb, however, is kimasu. Right away there is a discrepancy, as the first mora (Japanese syllable) is "ku" in the dictionary form, but "ki" in the masu form. The other formal forms seem to follow a typical pattern, as kimasu becomes kimasen in the negative, kimashita in the past, and kimasen deshita in the past negative. The short forms, however, show more irregularities. The short dictionary form, also used for the present tense in an informal situation, is kuru. The negative, however, is konai, and again we see the first mora change. The short past tense is kita, and the short past negative tense is konakatta. With these forms, the verb shows just how irregular it is. Here is a list of all the forms:
kimasu (present tense)
kimasen (present tense, negative)
kimashita (past tense)
kimasen deshita (past tense, negative)
Informal, short forms
kuru (present tense)
konai (present tense, negative)
kita (past tense)
konakatta (past tense, negative)
When your studies bring you to the more advanced conjugations, such as the conditional, you will see additional irregularities with this verb.
Irregular Verb: suru
This Japanese verb, meaning "to do" or "to make", is written as suru in its dictionary form. The present tense conjugation is shimasu, and just like kuru, we see an irregularity with the first mora right off the bat. Again like kuru, we see a familiar pattern with the formal forms, with shimasen in the negative, shimashita in the past tense, and shimasen deshita in the past negative. In the short form, we again see irregularities. The short dictionary form is suru. The negative short form is shinai, the past tense is shita, and the past negative is shinakatta. The verb suru is less irregular than kuru, but still has different forms. Here is a list of all the forms:
shimasu (present tense)
shimasen (present tense, negative)
shimashita (past tense)
shimasen deshita (past tense, negative)
Informal, short forms
suru (present tense)
shinai (present tense, negative)
shita (past tense)
shinakatta (past tense, negative)
As with kuru, there are other irregularities with this verb in the more advanced tenses, such as the conditional. The verb suru is very important to learn properly, because it is often seen in combination verbs, like benkyou suru (to study), and all of these use the irregular conjugations just like suru by itself.
Exception: aru and nai
The only two irregular verbs in Japanese are kuru and suru. However, there is one exception to this. The verb aru (to exist or to have) has an irregularity with its negative short forms, both present and past tense. The negative present tense is nai, and the negative past tense is nakatta. All the other forms of this verb are regular, and the only irregularities it has are there due to the evolution of the language from Ancient Japanese, so it is not typically grouped with the irregular verbs kuru and suru.