する: The Irregular Verb 'to do'

Japanese has just two irregular verbs, and する（"to do") is the one you will use the most. It is irregular in a very specific way: its stem is a single mora, and that mora changes its vowel depending on the form — し, さ, す, せ. There is no rule that predicts which vowel appears where, so you memorize the small paradigm. The payoff is enormous, because する is not only the verb "to do" but the engine that turns hundreds of nouns into verbs (勉強する, 電話する). Learn this one paradigm and all of them conjugate for free.

Why it can't be regularized

An English speaker's instinct is to treat する like a godan verb ending in -る and produce ×すます or ×すない. That never works, because する doesn't sit still on a single consonant the way godan verbs do. Instead its vowel jumps:

• し in most everyday forms — しない, します, した, して
• す in the dictionary and conditional — する, すれば
• さ before the causative and passive — させる, される
• せ in a few stiff, literary forms — せず, せよ

That is why it is "irregular": not because the endings are strange, but because the stem itself keeps borrowing a different vowel. Don't hunt for logic here — the four-vowel dance is the thing to memorize.

The core paradigm

These are the forms you need first. Note how し carries almost all the everyday weight:

Read that top-to-bottom and you'll see that する → します → しない → した → して all share the same し stem. The dictionary form is the odd one out with す. Get those six rows automatic and you can already say most of what a beginner needs to say.

する as a standalone verb

On its own, する means "to do" (and, with certain objects, "to play," "to wear," "to cost," or "to make a decision"). It takes a direct object marked with を:

That last example shows a use English never signals with "do": 〜する for "to cost." It's a reminder that する is a workhorse whose meaning bends to its object.

The real superpower: 〜する compound verbs

Here is where memorizing this one paradigm pays off many times over. Attach する directly to a noun — usually a two-kanji Sino-Japanese word — and you get a verb. 勉強（べんきょう, "study") + する = 勉強する ("to study"). 電話（でんわ, "telephone") + する = 電話する ("to phone"). The noun never changes; only the する part conjugates, following the exact paradigm above.

Because every する-compound conjugates identically, learning the する paradigm once effectively teaches you the conjugation of hundreds of verbs at a stroke. The compound-verb page covers how these behave in a sentence (including when を can split the noun from する), and suru-nouns lists the productive nouns that take it.

The さ and す branches

Two everyday forms reach past し for a different vowel, so it's worth meeting them now even though they belong to later topics:

• Potential ("can do") is suppletive: できる (dekiru). する has no regular potential form — you don't say ×しれる. Instead a completely separate verb, できる, fills the slot.
• Causative させる and passive される use さ, and the ば-conditional すれば uses す.

You'll also meet the せ stem in frozen, written-register forms like せず ("without doing," literary/formal) and the blunt imperative せよ ("do it," formal/written) — labeled that way because everyday speech uses しないで and しろ instead. You don't need to produce せず or せよ as a beginner; just recognize that せ is the fourth vowel this tiny stem can wear.

Common mistakes

Every error above comes from expecting する to behave like a normal -る verb and keep one fixed stem. It doesn't: the stem shifts, and the potential is a different verb entirely.

Key takeaways

• する is one of only two irregular verbs; its single-mora stem shifts between し, さ, す, せ.
• Memorize the everyday し forms: します, しない, した, して, しよう — they cover most of what you'll say.
• The dictionary and conditional use す (する, すれば); the causative/passive use さ (させる, される).
• Potential is suppletive: できる, never ×しれる.
• Attach する to a noun and you get a verb (勉強する, 電話する) — only the する part conjugates, so this one paradigm unlocks hundreds of compound verbs.