Learn Hardcore Russian: 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”).

How do the endings of пошёл show tense, gender, and number?

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.

Why is парк in the accusative case (в парк) rather than the prepositional (в парке)?

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).

How does the adjective большой agree with парк, and why doesn’t it change form?

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.

What is the role of обратно, and can it be replaced by назад or by using вернулся?

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.

If the friend were female, how would the sentence change?

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

Why is the possessive pronoun мой used, and how does it agree with друг?

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

What is the letter ё in пошёл, and how should it be pronounced?

The letter ё always indicates the stressed sound [o] with a [j] off-glide (like “yo”). In пошёл it marks both the stress and the correct pronunciation [pɐˈʂol]. Although many texts omit the two dots and write пошел, you still pronounce it as yo.