Special use of -이/가 and -을/를

Attention must be given to the choice of case marker or postposition for some verbs 
that appear to be transitive but in fact are intransitive in Korean. (A transitive verb is one that requires a direct object, and an intransitive verb is one that does not need a direct object.)

• a. Some verbs seem to be transitive in English but require the subject marker -이/가 in Korean: 필요하다 "to need," 되다 "to become," 있다 "to have."
• b. Some verbs seem to be intransitive but require the direct object marker 을/를 in Korean: 가다 "to go," 걷다 "to walk," 날다 "to fly," 다니다 "to attend."

(a)

• 필요하다: 외국어를 배우는 학생은 사전이 필요하다.
  => A student who studies a foreign
   language needs a dictionary.
  
• 되다: 도날드는 벌써 선생이 됐다.
  => Donald had already become a teacher.
  
• 있다/없다: 우리도 차가 없어요.
  => We too have no cars.

(b) 

• 걷다: 아이들이 길을 걷는다.
  => Children are walking on the road. (아이 child)
  
• 날다: 새가 하늘을 난다.
  => Birds are flying in the sky

This connective is similar to -(이)기 때문에 or -(이)어서 and is used in presenting a cause, condition, or conviction. -(이)라서 is used more colloquially than -(이)기 때문에 or -(이)어서.

From: http://www.language.berkeley.edu/korean/10/lesson01/01_grammar_only.htm