What's the difference between 할일없다 and 할일 없다?

 ANSWER: One means "unavoidable" or "inevitable," and the other means "have nothing to do."

할일없다, without a space, is an adjective that means "unavoidable," and 할일 없다, with a space, is a noun + adjective that means "have nothing to do." The 할일 is the noun, and the 없다 is the adjective. Since 할일 is a noun, you can use a subject marker and say 할일이 없다 instead of just 할일 없다.

However, the 할일없다, without a space, usually appears in the dictionary 하릴없다, which represents how it is pronounced. When it is pronounced, the ㄹ in 할 moves over to 일, resulting in /하릴/. Some may ask, "But wouldn't the 할일 없다, with a space, be pronounced the same as the 할일없다, without a space? The answer is, "No." 

If you want to say, "It is unavoidable," you say, /하릴없따/, but if you want to say, "I have nothing to do," you say, /할릴없따/. In other words, the difference in pronunciation is /하릴/ versus /할릴/. The reason the second one is pronounced as /할릴/ instead of /하릴/ is because a ㄴ was added to it to get 할닐, which then changes to /할릴/ because a ㄴ sound preceded by a ㄹ sound changes to a ㄹ sound.

So, if you want to say, 오늘 별로 할일 없어요 (Today, there is not much to do), be sure to pronounce the 할일 as /할릴/.