Learn Russian: Трое пастухов гоняют овец по полю. - Three shepherds are driving the sheep around the field.

Гонять is the multidirectional counterpart to гнать (to drive, chase, or herd). Since the shepherds are driving the sheep around the field (по полю)—moving in various directions rather than in one specific path towards a destination—the multidirectional гоняют is the correct choice.

Why does the sentence use трое пастухов instead of три пастуха?

Трое is a collective numeral. In Russian, collective numerals like двое, трое, and четверо are heavily preferred when counting groups of male persons (like пастух, shepherd). This emphasizes them acting together as a unit. After a collective numeral in the nominative case, the following noun takes the genitive plural (пастухов).

Why is the word for sheep овец?

Овец is the accusative plural of овца (sheep). Because sheep are animate (living beings), their accusative plural form matches their genitive plural form. Notice the inserted vowel е that appears between the final consonants of the stem to make it easier to pronounce (from овц- to овец).

How does по полю relate to the verb of motion in this sentence?