Learn Hardcore Russian: Мой друг живёт в соседнем доме. - My friend lives in the neighboring house.

друг is masculine singular.
The masculine singular form of мой is мой.
Other forms are:
- моя – feminine singular (e.g. моя сестра – my sister)
- моё – neuter singular (e.g. моё окно – my window)
- мои – plural for all genders (e.g. мои друзья – my friends)

So with друг, you must say мой друг.

What exactly is the verb form живёт? Which pronoun could replace мой друг here?

Живёт is the 3rd person singular present tense of the verb жить (to live).

The full present-tense paradigm is:

я живу – I live
ты живёшь – you live (informal singular)
он / она / оно живёт – he / she / it lives
мы живём – we live
вы живёте – you live (plural / formal)
они живут – they live

Since друг is grammatically masculine, you could replace Мой друг живёт в соседнем доме. with Он живёт в соседнем доме. (He lives in the neighboring house.)

Why is there a ё in живёт? Do Russians always write ё?

Phonetically, the stressed vowel here is [ё], so the correct spelling is живёт.

In practice:

In many printed and typed texts, ё is replaced by е, so you will often see живет.
Native speakers know from context and from the verb жить that it must be pronounced живёт, not живет with plain е.
In dictionaries, textbooks, and children’s books, ё is usually written to make stress and pronunciation clear.

As a learner, you should pronounce it as живёт even if you see живет.

Why is it в соседнем доме and not в соседний дом?

Because the sentence describes location (where someone lives), not movement (where someone is going).

For location in/at somewhere, Russian uses в
- prepositional case:
  - жить в доме – to live in a house
  - работать в офисе – to work in an office
For movement into somewhere, Russian uses в
- accusative case:
  - идти в дом – to go into the house
  - заходить в офис – to go into the office

So живёт в соседнем доме = lives in the neighboring house (static location), hence prepositional case.

What case is доме, and how is that form made from дом?

Доме is the prepositional case singular of дом (house).

Declension of дом (singular):

Nominative: дом – house (subject)
Genitive: дома – of the house
Dative: дому – to the house
Accusative: дом – house (direct object)
Instrumental: домом – with/by the house
Prepositional: (о) доме – in/about the house

After the preposition в for location, дом goes into the prepositional case: в доме. In the sentence, it is specified by an adjective, so we get в соседнем доме.

Why does соседнем end in -ем? How does it agree with доме?

Соседнем is the adjective соседний (neighboring, next-door) in the prepositional masculine singular form, agreeing with доме.

For a typical -ний adjective like соседний, the singular masculine/neuter endings are:

Nominative: соседний дом – neighboring house
Genitive: соседнего дома
Dative: соседнему дому
Accusative: соседний дом
Instrumental: соседним домом
Prepositional: (о) соседнем доме

Because доме is prepositional singular masculine, the adjective must also be prepositional singular masculine: соседнем.

Could I change the word order to В соседнем доме живёт мой друг? Does it change the meaning?

Yes, you can say В соседнем доме живёт мой друг. It is grammatically correct.

Both sentences describe the same situation, but the focus shifts:

Мой друг живёт в соседнем доме. – neutral; starts by introducing my friend, then says where he lives.
В соседнем доме живёт мой друг. – puts more emphasis on the location; for example, contrasting this house with other nearby houses, or answering Who lives in the neighboring house?

Russian word order is flexible; changing it usually affects emphasis or information structure rather than basic meaning.

What exactly does соседний дом mean? Is it literally the next house?

Соседний дом usually means the neighboring house or the house next door, i.e. a house that is directly adjacent or very close.

Typical nuances:

If you live in one house on a street, соседний дом is typically the house immediately next to yours (on either side).
It does not mean an apartment next door inside the same building; that would be соседняя квартира (the neighboring / next-door apartment).
In a broader sense, соседний can mean adjacent, neighboring for other things too: соседняя страна (neighboring country), соседний стол (the table next to us).

So в соседнем доме corresponds well to in the house next door / in the neighboring house.

Does друг mean only a male friend? How do you say a female friend or just friends?

Grammatically, друг is masculine and most commonly refers to a male friend.

Related forms:

Female friend: подруга
- Моя подруга живёт в соседнем доме. – My (female) friend lives in the neighboring house.
Plural friends (mixed or male): друзья
- Мои друзья живут в соседнем доме. – My friends live in the neighboring house.
Female friends (specifically female): подруги (plural of подруга)

In some very general or abstract contexts, друг can mean friend regardless of gender, but if you specifically mean a woman, подруга is normal.

Why is simple present живёт used, not something like is living? Does Russian have a separate continuous tense?

Russian has only one present tense form for actions and states: живёт covers both English lives and is living.

So Мой друг живёт в соседнем доме. can mean:

My friend lives in the neighboring house (permanent situation), or
My friend is living in the neighboring house (for now) (temporary), depending on context.

To show that something is temporary or ongoing, Russian normally adds adverbs or extra words, not a different verb form, for example:
Сейчас мой друг живёт в соседнем доме. – Right now my friend is living in the neighboring house.

Can you omit мой and just say Друг живёт в соседнем доме?

You can say Друг живёт в соседнем доме, and it is grammatically correct, but it sounds less natural in isolation.

Nuances:

Мой друг живёт в соседнем доме. clearly says my friend and sounds like normal, neutral speech.
Друг живёт в соседнем доме. feels more like a friend lives in the neighboring house or the friend lives in the neighboring house, and would usually refer to some friend already mentioned in the conversation or in a specific context.

In everyday Russian, people very commonly use possessive pronouns with words like друг, брат, сестра, etc., so Мой друг… is the default way to say My friend….

How is the whole sentence pronounced, and where is the stress?

Stresses:

Мой – [мой] (only one syllable, stressed)
друг – [друг] (one syllable, stressed)
живёт – stress on -вёт: живёт
соседнем – stress on се: соседнем
доме – stress on до: доме

Approximate pronunciation (with stress marks):
Мой друг живёт в соседнем доме. – [мой друг живёт в соседнем доме]

Note that в соседнем is often pronounced with assimilation: в + с → [фс], so it sounds like [ф соседнем доме] rather than a very clear в + с.