Мой друг пошёл обратно в большой парк.

пошёл is the past-tense, masculine singular form of the perfective, unidirectional verb пойти (to set off on foot once). Russian verbs of motion come in aspect-direction pairs:
• идти (imperfective, unidirectional) describes an ongoing movement,
• ходить (imperfective, multidirectional) describes habitual or repeated movement,
• пойти (perfective, unidirectional) marks a single completed start.
We use пошёл here because the sentence describes a completed action in the past (“he went”).

In Russian past tense, verbs take the suffix -л plus a gender/number ending:
• masculine singular: -л → пошёл
• feminine singular: -ла → пошла
• neuter singular: -ло → пошло
• plural: -ли → пошли
Since друг is a single male, the verb is пошёл.

When you express motion toward a place with в (or на), you answer куда? (“where to?”) and use the accusative case:
• в парк = “to the park” (direction)
To answer где? (“where?”) and describe location, you would use the prepositional:
• в парке = “in the park” (static location).

In the accusative singular for masculine inanimate nouns, adjectives take the same form as in the nominative.
• Nominative: большой парк
• Accusative (inanimate): большой парк
You would see большого in the genitive or большом in the prepositional, but not here.

обратно is an adverb meaning “back” or “backwards,” highlighting the return direction.
• пошёл обратно focuses on the act of going back on foot.
You can also say пошёл назад with nearly the same meaning; назад often directly follows the verb. Alternatively, вернулся is a single verb meaning “returned,” but пошёл обратно emphasizes the physical movement.

You’d switch to the feminine noun подруга, adjust the possessive pronoun and the verb ending:
Моя подруга пошла обратно в большой парк.

мой is the masculine singular nominative form of “my.” Possessive pronouns in Russian agree in gender, number, and case with the noun they modify. For example:
• masculine nom. sing.: мой друг
• feminine nom. sing.: моя подруга
• neuter nom. sing.: моё слово
• plural nom.: мои друзья