1. The pattern ‘-도 되다/괜찮다/좋다’
  2. The pattern `-(으)ㄹ 때'
  3. The connective `-기 때문에'


1. The pattern ‘-도 되다/괜찮다/좋다’ : (someone) may do (something)

On its own, the particle `-도' is used to express 'even if, even though'. See the following example first.
제가 내일 바빠도, 파티에 꼭 갈게요. Even though I am busy tomorrow,
I will go to the party.
한국말이 재미없어도 공부하겠어요. Even though Korean is not interesting,
I will study it.

However, if this pattern is used with the word '좋다, 괜찮다, 되다`, this pattern is used to ask for and giving permission. Here are some examples:
문을 열어도 괜찮아요 ? May I open the door ?
들어가도 괜찮아요/돼요/좋아요?
- 네, 들어와도 괜찮아요/돼요/좋아요. 		May I go in ?
- You may go in.
여기에서 담배 피워도 괜찮아요/돼요/좋아요?
- 네, 피워도 괜찮아요/돼요/좋아요. 		May I smoke here?
- You may smoke here.

For the negative answer to a request for permission, you have to use the pattern '-(으)면 안 되다.' '-으면 안 되다' is used after verb stems ending in a consonant except 'ㄷ', -면 안 되다' is used after verb stems ending in a vowel : an consonant 'ㄷ'.
 
들어가도 괜찮아요?
- 아니오, 들어오면 안 돼요.
- 네, 들어와도 괜찮아요/돼요/좋아요. 		May I go in ?
- You must not go in. (You shouldn't go in.)
- You may go in.
떠들면 안 돼요. You must not make noise.
(You shouldn't make noise.)
지각하면 안 돼요. Don't be late. (You shouldn't be late.)
 
2. The pattern `-(으)ㄹ 때' ; when/while

  The pattern `-(으)ㄹ 때' is used to express the time when something takes place or exists. When the two actions happen at the same time, past tense is not expressed with this pattern. But if the action of the `-ㄹ 때' clause happened before the action of the nexe clause, past tense should be expressed in the `-ㄹ때' clause. This pattern is used with all verbs and adjectives but `이다' can be used only in the past tense, not in the present tense. '-을때' is used after verb stems ending in a consonant, '-ㄹ때' is used after verb stems ending in a vowel.
그분이 떠날 때, 같이 갑시다. When he leaves, let's go together.
날씨가 좋을 때, 여행을 가겠습니다. When the weather is good,
I will take a trip.
제가 한국에 갔을 때, 날씨가 아주 추웠어요. When I went to Korea, it was very cold.
제가 학교에 갔을 때, 김선생님은 안 계셨어요. When I went to school,
Mr. Kim was not there.
내가 중학생이었을 때, 그곳에 갔어요. When I was a middle school student,
I went there.

  This pattern can be followed by any of the particles such as `-이/가, -을/를, -에, -도, -마다, -까지, -부터, etc..
학교에 갈 때가 되었어요. It's time to go to school.
(The time that we go to school came)
한국에 올 때마다 한국 음식을 먹어요. Whenever I come to Korea, I eat Korean food.
이 일은 시작할 때부터 끝날 때까지
기분이 좋았어요.		 From the time that I began this work,
to the time that I finished, I felt good.
3. The connective `-기 때문에' : so, therefore, because

  The connective `-기 때문에' is used to express cause and reason. When this connective is used, it most conveniently corresponds to 'so' in English.
시간이 없기 때문에 그곳에 갈 수 없었어요. I had no time, so I couldn't go there.
지금 너무 피곤하기 때문에 집에서 쉬고 싶어요. I am too tired now,
I want to take a rest at home.

  Contrary to the connective `-어서' which is used to express cause and reason only in the present form, this connective is used with all tenses.
비가 오기 때문에 안 가고 싶어요. Because it's rainning, I don't want to go.
비가 왔기 때문에 안 가고 싶어요. Because it rained, I don't want to go.
비가 오려고 하기 때문에 안 가고 싶어요. Because it will rain, I don't want to go.

  Sometimes it can be used with the sentent ending `-요' directlyto finish a sentence.
왜 등산을 안 갔어요 ?
- 비가 오기 때문에요. 		Why didn't you go climbing ?
- Because it's rainning.

  While this connective is used with verbs and adjectives, the form `- 때문에' (without the `-기') is used with nouns and has the same meaning.
비 때문에 안 가고 싶어요. Because of the rain, I don't want to go.
무엇 때문에 오셨어요 ? What makes you come here ?
(Because of what, did you come here ?)