Learn Turkish: Sonra dövüşüyorlar çünkü bu bir spor. - Then they spar because this is a sport.

The root is dövmek, which means to beat or hit. By adding the reciprocal suffix -üş, it becomes dövüşmek, meaning to hit each other. In this sentence, it is conjugated in the present continuous tense with -üyor and the plural -lar to mean they are fighting or sparring.

Why does the reciprocal suffix take the form -üş here instead of -ış or -iş?

The suffix changes according to the rules of 4-way vowel harmony. Because the vowel in the root döv- is 'ö', the suffix must use 'ü', making it -üş. If the root had an 'a' or 'ı', it would be -ış, like in yazışmak (to message each other).

Does dövüşmek specifically mean to spar in a sport?

Not exclusively. Dövüşmek generally means to fight physically with another person. In the context of a sport like boxing or martial arts, it translates well to sparring or having a match. If people were fighting in the street, you would also use dövüşmek.

Why do we need the word bir in bu bir spor?