Insert the ḍ-r-b root into the three textbook slots:
Word / formMeaning / explanation
مَضْرِبٌsingular, one striking place or time.
مَضْرِبَانِdual, two places or times.
مَضَارِبُbroken plural, multiple places or times.
The plural is not formed by simply adding -ūna or -āt. It changes the stem internally, including an alif, so it is a broken plural.