There is no definite "form" for verbs in Arabic. This goes to the fact that verbs have roots, for example: يذهب (Which is goes/to go) the -ing form of this verb is ذهاب (which is going)
But if you look at other verbs, such as يضرب/يقطع (to hit/to cut) Has the -ing form of قطع/ضرب
Other example is: يتنفّس (to breathe) which has the -ing form of تنفّس.
To learn the -ing form of a verb you need to know the infinitive form of it, or the root. Different verbs have different roots, and the form of the verb's root decides the -ing form of the verb you're looking for. If you need further information please tell me, and i will provide you with any type of information you need. I hope this helped