In short

Two first-person endings that sound alike but do different jobs. -(으)ㄹ게요 is a promise you make to the person in front of you - 제가 할게요 (Jega halgeyo, I’ll do it) - and it is never a neutral forecast. -(으)ㄹ래요 is what you feel like doing, or a casual invitation as a question: 커피 마실래요? (Keopi masillaeyo?, Want some coffee?).

The rule

Step 1 - -(으)ㄹ게요 is a commitment, first person only

You use -(으)ㄹ게요 to volunteer or reassure - to promise the listener that you will do something, decided right now in response to the situation:

Verb stem + -(으)ㄹ게요 -> 제가 할게요 (Jega halgeyo, I’ll do it), 다시 전화할게요 (dasi jeonhwahalgeyo, I’ll call you back).

Two hard limits follow from “it is a promise”:

• Only the speaker can promise. The subject is always I (저/제가). You cannot commit on someone else’s behalf, so a third-person 친구가 올게요 is wrong - use the plan/prediction ending -(으)ㄹ 거예요 (lesson 25) instead.
• It is not a forecast. The weather cannot promise you anything, so 내일 비가 올게요 is wrong; a prediction is 내일 비가 올 거예요 (Naeil biga ol geoyeyo, It’ll probably rain tomorrow).

Say it with a tensed ㄱ - 할게요 (halgeyo) is pronounced [할께요] - but spell it with the plain letter, -게요, exactly like the 거 in 거예요.

Step 2 - -(으)ㄹ래요 is your own will, and a casual invitation

-(으)ㄹ래요 expresses volition - what the speaker wants to do - and as a question it turns into a friendly shall we / do you want to:

• Statement (my will): 저는 안 갈래요 (Jeoneun an gallaeyo, I’d rather not go), 저는 커피 마실래요 (Jeoneun keopi masillaeyo, I’ll have coffee).
• Question (casual invite / asking your wish): 우리 같이 갈래요? (Uri gachi gallaeyo?, Shall we go together?), 뭐 먹을래요? (Mwo meogeullaeyo?, What do you want to eat?).

Register limit: -(으)ㄹ래요 is casual - fine with friends, peers, and juniors, but too blunt for a boss or an elder. To offer something upward, switch to the honorific -(으)시겠어요?: 가시겠어요? (gasigesseoyo?, would you like to go?). The (으) in it is the same 받침 rule as everywhere else here - 받으시겠어요? (badeusigesseoyo?) after a consonant - and the suffix inside it is the -(으)시- of lesson 22, which is also where the suppletive honorific verbs (드시다, 계시다) come from.

Step 3 - Same -(으)ㄹ attachment, and the negatives

Both endings attach with the ordinary 받침-driven -(으)ㄹ choice: vowel + -ㄹ게요/-ㄹ래요, 받침 + -을게요/-을래요, and a ㄹ-stem just uses its own ㄹ with no 으 (만들다 -> 만들게요, mandeulgeyo).

Negatives sit in their natural slots. For volition, put 안 in front: 안 갈래요 (an gallaeyo, I won’t / I’d rather not). For a promise not to do something, use long-form negation: 가지 않을게요 (gaji aneulgeyo, I won’t go), 다시는 늦지 않을게요 (dasineun neutji aneulgeyo, I won’t be late again).

Forms & patterns

-(으)ㄹ게요 - the allomorph table:

-(으)ㄹ래요 - the allomorph table:

The four-way contrast (all on 하다):

Examples

Common mistakes

• 할께요 -> 할게요 (halgeyo) - The tense sound [께] is real, but the standard spelling keeps the plain ㄱ in the ending -게요, just as 거예요 keeps a plain 거. Say [할께요], write 할게요.
• 내일 비가 올게요. (Naeil biga olgeyo.) -> 내일 비가 올 거예요. (Naeil biga ol geoyeyo.) - -(으)ㄹ게요 is a personal promise; rain cannot promise you anything. A forecast takes the prediction ending -(으)ㄹ 거예요.
• 친구가 올게요. (Chinguga olgeyo.) -> 친구가 올 거예요. (Chinguga ol geoyeyo.) - You can only commit on your own behalf. A third-person subject means you are guessing, so use -(으)ㄹ 거예요.
• 사장님, 같이 갈래요? (Sajangnim, gachi gallaeyo?) -> 사장님, 같이 가시겠어요? (Sajangnim, gachi gasigesseoyo?) - -(으)ㄹ래요? is too casual for a superior; offer upward with the honorific -(으)시겠어요?.
• 읽게요. (ilgeyo.) -> 읽을게요. (ilgeulgeyo.) - After a 받침 the ending is -을게요, not a bare -게요. The (으) is the same 받침 rule you use everywhere.

Tips

• Two questions sort them out. Am I promising you? -> -(으)ㄹ게요. Am I telling you what I feel like? -> -(으)ㄹ래요. And did I already have this plan? -> -(으)ㄹ 거예요 - a report, not a promise.
• -게요 reacts, -ㄹ 거예요 reports. You reach for 할게요 the moment you decide to help; 할 거예요 describes a plan that was already in place. The hearer’s presence is what triggers -게요.
• Keep -(으)ㄹ래요 among equals. It is warm and casual - great with friends, wrong with a boss. Trade it for -(으)시겠어요? the instant you are speaking upward.
• Sound in, spelling out. After the ㄹ of -(으)ㄹ게요 and -(으)ㄹ 거예요 the next consonant is tensed - [할께요], [할 꺼예요] - yet you spell them with the plain letters -게요 and 거예요. Never spell what you hear. (-(으)ㄹ래요 has nothing to tense: [갈래요] sounds as written.)