In everyday informal conversations, you can omit object particles -를/-을 if the object is obvious. But such omissions, if overused, may make you sound like a baby. Instead of omission, to specify the object quickly, Koreans often contract (shorten) -를 by pronouncing ㄹ at the final position of the preceding noun. 

  • 바다를 => 바달  
  • => 널 
  • 하기 => 하길 

On the other hand, -을 can not be contracted because the noun already has a consonant, except for frequently used nouns like (thing) or 무엇 (what); for these popular nouns, one can supersede the final ㅅ with the more powerful ㄹ.  

  • 이것=> 이걸 
  • 무엇 => 무얼 / 뭘 

All consonants are created equal, but some consonants are more equal than others.^^

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