I'll give you some examples. 자리가 없어.= There are no seats(informal) - You can use it saying to your friends or close someone/ commonly use 자리가 없다 =There are no seats (informal, formal)- It used to formal and in formal both diffends on intonation / 없어 is more common for communication / You can see 없다 a lot in sentencs as you read a book or article. 자리가 없냐? = Are there no seats ? (very informal) - It used to be only quesiton which has a rising intonation. It sounds like a full of mischief. You can use it for only your close frind. 자리가 없지 = There are no seats (informal) - You and your friend got on a bus in the rush hour. You already forecasted " there isn't no seat on a bus". Your friend asks "자리가 없어?" then you answer "자리가 없지" (You've forcasted that) >>I tryed to explain well though, my anwer might be confusing from my bed English.