In short

하다 (hada, to do) glues onto a plain noun and turns it into a verb: 공부 (study) -> 공부하다, 전화 (phone) -> 전화하다. It is the single most productive word in Korean, and it conjugates by one irregular rule you only ever have to learn once: 하 + 여 -> 해.

The rule

Step 1 - Noun + 하다 makes a verb

Take a noun that names an activity and attach 하다 with no space. The whole thing behaves as one ordinary verb.

Noun + 하다 -> Noun + 해요

• 공부 (gongbu, study) -> 공부해요 (gongbuhaeyo, [I] study)
• 일 (il, work) -> 일해요 (ilhaeyo, [I] work)
• 전화 (jeonhwa, a phone call) -> 전화해요 (jeonhwahaeyo, [I] call)
• 운동 (undong, exercise) -> 운동해요 (undonghaeyo, [I] exercise)

The mechanism is still open: Korean applies it to new loanwords the moment they arrive. 체크하다 (chekeuhada, to check), 스캔하다 (seukaenhada, to scan), 클릭하다 (keullikada, to click) are all built the same way. If you know the noun, you can usually build the verb.

Step 2 - The 여 불규칙, learned once for hundreds of verbs

Every other verb picks -아요 or -어요 by its stem vowel (lesson 10). 하다 belongs to a class of its own: its stem 하- takes -여요, and 하 + 여 contracts to 해. *하아요 does not exist at all; the uncontracted 하여요 is a real form, but a stiff literary one, so in ordinary speech and writing always write 해요.

That one contraction propagates through the whole paradigm, because everything built on the 아/어 form inherits it:

해요 (haeyo) - 했어요 (haesseoyo) - 해서 (haeseo) - 해야 (haeya) - 해 주세요 (hae juseyo)

The forms built on the stem instead, not on the 아/어 form, are perfectly regular: 하세요 (haseyo), 할 거예요 (hal geoyeyo), 하면 (hamyeon), 하고 (hago). And because this applies to 하다 itself, it applies automatically to 공부하다, 전화하다, 사랑하다 and every other member: 공부해요, 전화했어요, 사랑해서.

Step 3 - The separable object: 공부해요 vs 공부를 해요

The noun inside a 하다 verb is still a noun, so it can be pulled out and marked as a real object with 을/를:

Noun + 하다 (solid, the default) <-> Noun + 을/를 + 하다 (split)

• 공부해요 (gongbuhaeyo) <-> 공부를 해요 (gongbureul haeyo)
• 전화했어요 (jeonhwahaesseoyo) <-> 전화를 했어요 (jeonhwareul haesseoyo)

Two things decide which you write:

• Solid is the neutral form. Use it unless something forces a split.
• An adverb or a modifier between the noun and 하다 forces the split, because you cannot insert anything inside a single word: 공부를 열심히 해요 (Gongbureul yeolsimhi haeyo, [I] study hard), 전화를 두 번 했어요 (Jeonhwareul du beon haesseoyo, [I] called twice).

The split also adds a light emphasis on the noun. Note the spacing follows automatically: once 을/를 is there, 해요 is a separate word and takes a space.

One clause holds one object. If the sentence already has an object, keep 하다 solid: 한국어를 공부해요, never *한국어를 공부를 해요.

Step 4 - 하다-adjectives

Not every 하다 word is an action. Many are descriptive verbs (lesson 11) - they describe a state, and they simply happen to end in 하다:

조용하다 (joyonghada, to be quiet), 깨끗하다 (kkaekkeutada, to be clean), 필요하다 (piryohada, to be necessary), 미안하다 (mianhada, to be sorry), 중요하다 (jungyohada, to be important)

They conjugate identically - 조용해요, 깨끗했어요 - but they carry the restrictions of every descriptive verb:

• No 을/를 split. There is no object to pull out: 조용을 해요 is not Korean. The related adverb is 조용히 (joyonghi): 조용히 해요 ([I] keep quiet*).
• No imperative or proposal. *깨끗하세요 is wrong; you say 깨끗하게 하세요 (kkaekkeutage haseyo, make it clean).
• 필요하다 marks what is needed with 이/가, not 을/를: 돈이 필요해요 (Doni piryohaeyo, [I] need money), never *돈을 필요해요.

Quick test: if Noun + 을/를 + 해요 is possible, it is an action verb; if not, it is an adjective.

Step 5 - 하다 vs 되다

Many nouns take both 하다 and 되다 (doeda, to become / to get done). The pair splits the work:

• 하다 - somebody does it. 제가 회의를 시작해요. (Jega hoeuireul sijakaeyo.) - I start the meeting.
• 되다 - it happens, with no agent named. 회의가 시작돼요. (Hoeuiga sijakdwaeyo.) - The meeting starts.

되다 is not irregular - it simply contracts: 되 + 어요 -> 돼요 (dwaeyo), 되 + 었어요 -> 됐어요 (dwaesseoyo), and the contracted spelling is the normal one. Common pairs: 시작하다/시작되다, 준비하다/준비되다, 걱정하다/걱정되다, 취소하다/취소되다. Note that 되다 takes 이/가 on what English would call the object. This is the doorway to the passive - full treatment in lesson 55.

Step 6 - Spacing

명사+하다 is one word: 공부하다, 운동하다, 전화하다. Writing *공부 하다 with a space is a spelling error. The space appears only when a particle comes between them (공부를 해요) or when 하다 is the free verb to do (뭐 해요? - What are you doing?).

Forms & patterns

하다 in full - the paradigm you reuse for every 하다 verb:

Noun + 하다, solid and split:

하다-adjectives - same endings, different rules:

하다 vs 되다:

Examples

Common mistakes

• 하아요. (Haayo.) -> 해요. (Haeyo.) - 하다 does not take -아요. Its stem takes -여요, and 하 + 여 contracts to 해.
• 저는 공부 해요. (Jeoneun gongbu haeyo.) -> 저는 공부해요. (Jeoneun gongbuhaeyo.) - with no particle between them, the noun and 하다 are a single word and take no space.
• 조용을 해요. (Joyongeul haeyo.) -> 조용히 해요. (Joyonghi haeyo.) - 조용하다 is a descriptive verb, so there is no object noun to split off. Use the adverb 조용히.
• 저는 돈을 필요해요. (Jeoneun doneul piryohaeyo.) -> 저는 돈이 필요해요. (Jeoneun doni piryohaeyo.) - 필요하다 describes a state, not an action; what you need is its subject, marked with 이/가.
• 제가 회의를 시작돼요. (Jega hoeuireul sijakdwaeyo.) -> 제가 회의를 시작해요. (Jega hoeuireul sijakaeyo.) - 되다 never takes an object. If you name the person doing it, the verb must be 하다.

Tips

• Learn 하다 once, get hundreds of verbs free. 해요, 했어요, 해서, 해야, 하세요, 할 거예요 is the whole story. Every 명사+하다 verb - including ones invented last week - inflects exactly like this, so the 여 불규칙 is the highest-return five minutes in A1.
• The 을/를 test tells you the word class. If Noun + 을/를 + 해요 sounds right (공부를 해요, 운동을 해요), it is an action verb. If it does not (*조용을 해요, *필요를 해요), it is a 하다-adjective and follows lesson 11’s rules instead.
• 하다 = I do it, 되다 = it gets done. English uses one verb for both (the meeting starts / I start the meeting); Korean makes you choose, and 되다 puts 이/가 on the thing affected.
• Negation goes inside the compound. With 안, the split is obligatory: 공부 안 해요 (gongbu an haeyo), not *안 공부해요. The full picture is next door in lesson 14.