The violence pit pro-Russian separatists against Ukrainian forces and those who support the government in Kiev.
A friend of mine says pit is used as a past tense verb in this sentence. What is the present tense form of pit?
This is an example of usage differing in Britain and the USA. For some reason, most Americans don't change the verb form for the past tense of certain short verbs ending in -it. So fit, spit, shit, slit and pit are thus formed both in the present and the past tense as far as Americans are concerned, whereas people from Britain would almost always use fitted, spat, shat, slitted and pitted for the past tense. However, even in Britain hit, split and quit follow the American pattern (though quitted is also used); and Americans do say sat rather than sit.
Your friend is correct that pit is being used as a past tense in the example, but by most standards it is being used incorrectly. Dictionaries (e.g. Collins) mostly say that the verb is pit, and the past tense is pitted. It is clear that the past tense is intended, (the present tense would be pits), and there certainly are those who think pit, used transitively, retains its irregular past pit; but the word itself is so rare that it is being regularized by sheer unfamiliarity.