Learn Hardcore Russian: Мой друг может отремонтировать старый дом.

друг (friend) is a masculine noun.
The masculine form of my is мой.
The feminine form is моя (used with feminine nouns like мама → моя мама).

So:

мой друг = my (male) friend
моя подруга = my (female) friend

Does друг always mean a male friend? What if the friend is a woman?

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

For a female friend, Russians normally use:

подруга (feminine noun) → моя подруга = my (female) friend

In some contexts (like addresses such as Дорогие друзья! – Dear friends!), друг in the plural (друзья) can be gender‑neutral and include both men and women.
But мой друг almost always implies the friend is male.

What case is старый дом in, and how do I recognize it?

Старый дом is in the accusative case, because it is the direct object of the verb отремонтировать (to repair).

дом is a masculine inanimate noun.
For masculine inanimate nouns, the accusative form = nominative form.
- Nominative: старый дом (subject)
- Accusative: старый дом (object) – same form

So even though it looks nominative, its function in the sentence (what is being repaired) makes it accusative.

Why is the adjective старый also in that form? How does it agree with дом?

Adjectives in Russian agree with the noun in:

gender
number
case

Here:

дом – masculine, singular, accusative, inanimate
старый – masculine, singular, accusative, inanimate

For masculine inanimate singular, the adjective ending -ый is used in both nominative and accusative:

Nominative: старый дом
Accusative (inanimate): старый дом

So старый matches дом perfectly in gender/number/case.

Why is it может отремонтировать and not just ремонтирует?

Может отремонтировать expresses ability / possibility: can repair / is able to repair.

может is 3rd person singular of мочь (can, to be able).
ремонтирует is 3rd person singular of ремонтировать (repairs / is repairing).

Compare:

Мой друг может отремонтировать старый дом.
My friend can repair the old house. (He has the ability / it is possible.)
Мой друг ремонтирует старый дом.
My friend is repairing the old house. (He is actually doing it now or regularly.)

So может + infinitive focuses on can / is able to, not on the current ongoing action.

Why is the verb отремонтировать (with от-) used instead of ремонтировать?

This is an issue of aspect, very important in Russian verbs.

ремонтировать – imperfective aspect
- Process, repeated action, or general activity
- to be repairing, to repair (as an activity), to do repairs
отремонтировать – perfective aspect
- Completed action with a result
- to repair completely / to get (something) repaired

In this sentence:

Мой друг может отремонтировать старый дом.
Emphasizes: he can successfully complete the repair and get the house into good condition.

If you said:

Мой друг может ремонтировать старый дом.
This sounds more like he is capable of doing repair work on the house (the process), not necessarily bringing it to a finished, fixed state.

Can может here mean may (permission), or only can (ability)?

In Russian, может (from мочь) mostly expresses:

ability / possibility: can / is able to / is able to possibly do it

Permission (may) is usually expressed with:

context and можно (impersonal):
- Моему другу можно отремонтировать старый дом.
  My friend is allowed to repair the old house.

So in Мой друг может отремонтировать старый дом, the natural meaning is can / is capable of, not is allowed to.

Where is the stress in each word, and how are they roughly pronounced?

Approximate stresses and pronunciations:

Мой – one syllable, stressed: мой
Roughly: moy (like English boy, but with m)
друг – one syllable, stressed: друг
Roughly: droog (rolled or tapped r, final g more like hard g)
может – stress on the first syllable: мО-жет
Roughly: MO-zhet (zh as in measure)
отремонтировать – stress on -ти-: от-ре-мон-тИ-ро-вать
Roughly: at-ree-mon-TEE-ra-vatʲ
старый – stress on first syllable: стА-рый
Roughly: STAH-riy (soft r and final y like in silly)
дом – one syllable, stressed: дом
Roughly: dom (close to dome but with a pure o and no glide)

Russian has no a/the. How do I know if this means an old house or the old house?

Russian has no articles, so дом can mean:

a house
the house

The difference is determined by context, not by a separate word:

If this is the first time we mention the house:
→ Usually translated as an old house.
If both speakers already know which house is meant:
→ Usually translated as the old house.

So старый дом on its own is neutral; English a/the is added by the translator depending on context.

Is this word order fixed, or can I change it?

Мой друг может отремонтировать старый дом.

Russian word order is fairly flexible, but the given order is neutral and natural:

Мой друг – topic / subject
может отремонтировать – predicate (what he can do)
старый дом – object

You can change word order for emphasis or style:

Старый дом может отремонтировать мой друг.
Emphasis on старый дом (It is the old house that my friend can repair).
Мой друг старый дом может отремонтировать.
Emphasis on what he can do with the old house (spoken, more emotional).

But for a simple, neutral statement, the original order is best for a learner.

How would I say My friend cannot repair the old house?

You negate может with не:

Мой друг не может отремонтировать старый дом.
= My friend cannot repair the old house.

The pattern is:

не
- может
  - infinitive
    (не может сделать, не может понять, не может починить, etc.)

How would the sentence change in the plural: My friends can repair old houses?

You must change:

мой друг → plural мои друзья
может → plural могут
старый дом → plural старые дома

Result:

Мои друзья могут отремонтировать старые дома.
= My friends can repair old houses.

Agreements:

мои (plural) with друзья (plural)
старые (plural) with дома (plural)
могут (3rd person plural) with друзья as the subject.