-기 때문에 is used in formal writing -아서 = 아 + 서 아 is added when the vowel of the verb stem is ㅗ(오) or ㅏ(아), for other verbs(except for irregular verbs) you use 어. A couple of examples 좋다 → the stem is 좋, its vowel is ㅗ. So you should pick 아 up. 자라다 → the stem is 자라. Here you should see the final letter, you pick 아 up. -워서 = 워 + 서 When [ㅂ다]-shaped verbs combine with a conjugational ending(서, 도, 야, 요, ㅆ), ㅂ becomes 우 and then you can figure out you should use 어 from above. e.g. 귀엽다 ⇒ 귀여워서, 덥다 ⇒ 더워서 -해서 = 해 + 서 [-하다]-shaped verbs are irregular verbs. 나 너 좋아해 ⇒ 나 너 좋아해서 blablabla 걔 착해 ⇒ 걔 착해서... About (3), (4), I'd like you to check this : http://www.italki.com/answers/question/55204.htm