Verb(ends with a vowel) + ㄹ게요
Verb(ends with a consonant) + 을게요

-을게요 is a future tense that means ‘I will’.  We often get confused by -을 거예요 because -을 거예요 is also future tense. But they are very different.

-을게요 only can be with the first person as a subject while -을 거예요 can be with the first, second  and third person.  And 을게요 only is used as a statement while -을 거예요 can be used for a statement, question. In addition, 을게요 only can be with verbs while 을 거예요 can be with adjectives when it has a meaning of prediction.  For example,

을 거예요	제가 내년에 한국에 갈 거예요. I am going to Korea next year. (my plan: statement)
	민수 씨, 내년에 한국에 갈 거예요? Minsu, are you going to Korean next year? (your plan: question)
	민수 씨가 내년에 한국에 갈 거예요.  Minsu is going to Korea next year. (his/her plan : statement) 내일 비가 안 올 거예요. It will not rain tomorrow. (prediction) 지금 한국 서울이 아주 추울 거예요. It will be very cold in Seoul, Korea. (prediction 춥다 : adjective)
을게요	Q: 민수 씨, 언제 올 거예요?  Minsu, when are you coming? A:  금방 갈게요. I will be there soon. (my plan: commitment)

When you use -을게요, there must be a listener who you give a commitment because -을게요 expresses ‘commitment‘. So it is not just a future tense, but it is a ‘promise‘.  For example,

A : 민수 씨, 숙제를 언제 보낼 거예요? When are you going to send homework? B : 내일 보낼 거예요. (I am going to send it tomorrow.)	A will be just a friend who is curious about if B send homework. B tells A that B is going to send it tomorrow, but it does not matter to A.
A: 민수 씨, 숙제를 언제 보낼 거예요? B: 내일 보낼게요. (I will send it tomorrow.)	A will be a teacher who asks B when he is going to send the homework. B promises A he will send it tomorrow.

Here are more examples.

Q : 민수 씨, 제가 생일 선물로 쿠키를 보냈어요.  I sent some cookies for your birthday gift.
A : 고마워요. 잘 먹을게요!  Thanks! I will enjoy them. 

Q: 내일까지 끝낼 수 있어요?  Can you finish this by tomorrow?
A: 네, 내일까지 끝낼게요. Yes, I will finish it by tomorrow.

Q: 불고기를 먹고 싶어요. 그런데 불고기를 만들 줄 몰라요.  I want to eat Bul-gogi, but I don’t know how to make it.
A: 제가 만들 줄 알아요. 오늘 저녁에 만들어 줄게요. I know, I will make it for you this evening. 

Q: 내일 저녁 6시에 공항에 도착할 거예요. I will arrive at the airport at 6 pm tomorrow.
A: 네, 그럼 공항 안에서 기다릴게요. I will wait for you inside the airport. 

Q: 민수 씨, 제가 오늘 좀 아파요. 그래서 학교에 못 가요.  Minsu, I am sick today, so I can’t go to school.
A : 걱정하지 마세요. 제가 내일 제 공책을 보여 줄게요.  Don’t worry! I will show you my note tomorrow.

Q: 무서워요!  I am scared!
A: 무서워하지 마세요. 제가 지켜줄게요.  Don’t be scared! I will protect you! 

Here is another notice!

A lot of Koreans write 을게요 as 을께요, or 을 거예요  as 을 꺼예요 however 을께요 and 을 꺼예요 are wrong!  The pronunciation of 을게요 is /을께요/ and 을 거예요 is /을 꺼예요/, so many Koreans get confused by the pronunciation and spelling.  If you see someone writing it as 을께요or 을 꺼예요, please correct them! 🙂