Irregular and する verbs

Now that we've covered the most common verb stems, we can cover the irregular verbs. Japanese only has two major irregular verbs, but they're very common. Their conjugations are completely irregular. Note that there are some other exceptions and corner cases in some other verbs (we’ve already seen 問うた), but these two are completely different and live in an entire class of their own.

する
to do

来る
to come

Their conjugations are as such

する		to do
した		did
しない		to not do
せよ or しろ	do!

くる		to come
きた		came
こない		to not come
こい		come!

With する, we can introduce a new class of verbs: the する verbs. In Japanese you can turn a lot of nouns into verbs by simply adding する after them. This is not allowed with all nouns, but a large part of words will show up in the dictionary as “する verb” too. This is especially common between two-kanji words that originate from Chinese.

料理
cuisine

料理する 
to cook (lit: “to do cuisine”)

They mostly mean the same as their non-verb construction, but grammatically the noun becomes for all intents and purposes a real verb.

Consider these sentences

日本語を勉強する
to study Japanese

日本語の勉強をする
to study Japanese (lit: “to do the study of Japanese”)

*日本語を勉強をする  
(ungrammatical nonsense)

The first treats 勉強 as a verb, and 日本語を as its object. The second considers 勉強 a noun (modified by 日本語) and it is the object of the individual verb する. The third sentence is ungrammatical, because it tries to force two disconnected objects and simply doesn’t work.

While we can translate the first and second sentences in two slightly different ways to accentuate the difference between 勉強する and 勉強をする, realistically the meaning is pretty much the same.