Learn Hardcore Russian: Мой друг идёт в парк со мной.

Because with motion toward a destination you use в + accusative: в парк (into the park). For location (being inside), you use в + prepositional: в парке (in the park).

What case is парк here, and why doesn’t it change form?

It’s accusative singular of an inanimate masculine noun ending in a consonant. Inanimate masculine nouns of this type have the same form in nominative and accusative: парк → парк.

Why со мной and not с мной?

The preposition с becomes со before certain consonant clusters to make pronunciation easier. Since мной starts with the cluster мн-, the standard form is со мной. You may also see the slightly more formal/poetic со мною.

What case is мной, and what are the forms of я?

мной is the instrumental singular of я, used after с/со to mean “with.” Quick forms of я: nominative я, genitive меня, dative мне, accusative меня, instrumental мной/мною, prepositional мне.

Why идёт and not ходит, пойдёт, or едет?

идёт: one-direction, on foot, in progress now (or arranged near future).
ходит: multi-direction/habitual (goes regularly; back-and-forth).
пойдёт: perfective future “will go (set off).”
едет: by vehicle (is going/driving/riding).

Can идёт refer to the near future?

Yes. With a time word, the present can express a planned future: Мой друг идёт завтра в парк = “My friend is going (tomorrow).” To emphasize the future action itself, use пойдёт/поедет.

Does идти always mean “on foot”? What if we’re driving?

Literally, идти is on foot. For transport, use ехать/поехать. Colloquially, пойти can also mean “to attend/go (to an event/place)” regardless of how you get there (e.g., Пойдём в кино).

How is идёт pronounced, and why is the dot on ё important?

идёт is pronounced roughly “ee-DYOT,” and ё is always stressed and pronounced “yo.” In real-world texts, ё is often printed as е (идет), but you still pronounce it “идёт.”

Any other pronunciation tips for the sentence?

друг ends with a devoiced г: it sounds like [druk].
в парк often sounds like [ф парк] due to devoicing of в before a voiceless consonant.
The д in идёт is soft (palatalized).
A simple approximation: “MOY druk i-DYOT v park sa MNOY.”

Why мой and not моя?

Agreement. друг is masculine, so you use masculine мой. For a female friend: моя подруга. For plural “my friends”: мои друзья.

Is друг specifically a male friend? What about a female friend?

Yes, друг is grammatically masculine (male friend). A female friend is подруга: Моя подруга идёт в парк со мной. For “friends” (any gender, plural): друзья.

Can I change the word order?

Yes, Russian word order is flexible:

Мой друг идёт со мной в парк.
Мой друг со мной идёт в парк.
Со мной мой друг идёт в парк. All are grammatical; moving pieces changes emphasis (earlier = more emphasis).

Why в парк and not на парк?

With парк, Russian uses в (you go into an area): в парк / в парке. Many places have fixed prepositions: на пляж, на стадион, на работу, but в парк.

What’s the difference between в парк and к парку?

в парк: into the park (entry; inside is the destination).
к парку: toward/up to the park (vicinity), not necessarily entering.

How would I say “My friends are going to the park with me”?

Мои друзья идут в парк со мной. If by vehicle: Мои друзья едут в парк со мной.

What’s the present-tense conjugation of идти?

я иду
ты идёшь
он/она идёт
мы идём
вы идёте
они идут

Could I omit со мной?

Yes: Мой друг идёт в парк. That simply states the destination and says nothing about company.

How would the cases change if I center “with my friend” instead (e.g., “I am going to the park with my friend”)?

You’d use the instrumental for “friend”: Я иду в парк со своим другом (or …с моим другом). Note друг → другом, and the possessive agrees: моим/своим.

Can I add вместе for “together”?

Yes, to emphasize togetherness: Мой друг идёт в парк вместе со мной. со мной already implies “with me,” but вместе adds extra emphasis.