Learn Hardcore Russian: Их племянник иногда помогает им по дому. - Their nephew sometimes helps them around the house.

Их is invariable: it does not change for gender, number, or case.
- их брат – their brother
- их сестра – their sister
- с их друзьями – with their friends
In this sentence их shows who the nephew belongs to: их племянник = their nephew (nephew of them).

What case is племянник in, and why?

Племянник is in the nominative singular case.
The nominative is used for the subject of the sentence – the person or thing doing the action.

Here, их племянник (their nephew) is doing the action помогает (helps), so племянник must be nominative.

Why is it помогает (singular) and not помогают (plural)?

The verb must agree with the subject, not with the other people in the sentence.

Subject: их племянник – one person → 3rd person singular
So the verb is: он помогает → их племянник помогает

The pronoun им (to them) is an indirect object, not the subject, so it does not control the verb form.

What’s the difference between помогает, помогать, and помочь?

Помогать – infinitive, imperfective aspect: to help (process, repeated or general action).
Помогает – 3rd person singular present of помогать: he/she helps, he/she is helping.
Помочь – infinitive, perfective aspect: to help as a single, completed action (to help once / to manage to help).

In this sentence we talk about something done sometimes, regularly, so Russian uses the imperfective:
Их племянник иногда помогает им по дому.

What case is им and why is it used here?

Им is dative plural of the pronoun они (they).

The verb помогать (to help) in Russian takes its indirect object in the dative case – the person receiving the help:

помогать кому? – to help whom? (in English who(m), but grammatically like to whom)
помогать маме – to help mom
помогать друзьям – to help (the) friends
помогать им – to help them

So им = to them.

What is the difference between их, им, and ими?

All three are forms related to они (they), but with different roles:

их – their / them (genitive/accusative, and also possessive; invariable)
- их племянник – their nephew
- Я вижу их. – I see them.
им – to them (dative plural)
- Я помогаю им. – I help them.
ими – by them / with them (instrumental plural)
- Я доволен ими. – I am pleased with them.

In our sentence: им is used because помогать requires the dative.

What does по дому mean literally, and why is по used?

Literally, по дому is around the house / about the house / in the house area.
Preposition по + dative can mean movement or activity within a space, throughout a place, or in relation to something.

In everyday speech, по дому in this context has the idiomatic meaning:

помогать по дому = to help with housework / to help around the house

So по дому is not about direction (like English towards); it’s an established phrase meaning helping with household tasks.

Why is дом in the dative case in по дому?

Because the preposition по in this meaning (activity within an area, “around the house”) governs the dative case.

Declension of дом (singular, main cases):

Nominative: дом (house)
Genitive: дома (of the house)
Dative: дому (to/for the house)
Accusative: дом
Instrumental: домом
Prepositional: (о) доме

After по in this expression we use дому (dative): по дому.

What is the difference between по дому and дома?

дома = at home (location)
- Он дома. – He is at home.
по дому = around the house, usually in the sense of doing housework / helping with chores
- Он помогает по дому. – He helps around the house (with housework).

So:

Он дома. – He is at home (location).
Он помогает по дому. – He helps with things that need to be done at home (tasks, chores).

Where can иногда be placed? Is its position fixed?

Иногда is an adverb (sometimes) and is fairly flexible in word order. All of these are possible and natural, with slight differences in emphasis:

Их племянник иногда помогает им по дому. (neutral, most typical)
Иногда их племянник помогает им по дому. (emphasis on sometimes)
Их племянник помогает им иногда по дому. (less common, slight emphasis on sometimes).

The version in the sentence is the most neutral and common: иногда just before the verb.

Is племянник the same as внук?

No, they are different:

племянник – nephew (son of your brother or sister, or of your spouse’s brother/sister).
внук – grandson (son of your son or daughter).

So их племянник = their nephew, not grandson.

How would the sentence change if the nephew were female (niece)?

You only need to change the noun:

Их племянница иногда помогает им по дому.

Notes:

Племянница is feminine;
The verb помогает stays the same (3rd person singular doesn’t show gender in the present tense);
The rest of the sentence is unchanged.

How would you say Their nephews sometimes help them around the house?

You make the subject plural and match the verb:

Их племянники иногда помогают им по дому.

Changes:

племянник → племянники (plural)
помогает → помогают (3rd person plural to agree with племянники).
им stays the same: dative plural = to them.

Is it possible to change the word order, for example: Их племянник им иногда помогает по дому?

Yes, Russian word order is flexible. For example:

Их племянник иногда помогает им по дому. (neutral)
Их племянник им иногда помогает по дому. (slight emphasis on им – to them)
Им их племянник иногда помогает по дому. (stronger emphasis that it’s to them that their nephew helps).

All are grammatically correct; the original order is the most neutral and standard for learners.

Any pronunciation tips for this sentence?

Key stress patterns:

Их племя́нник иногда́ помога́ет им по до́му.

Stressed syllables in bold (showing with capitals here for clarity):

племЯНник – stress on -ян-
иногДА – stress on -да
помогАет – stress on -га-
ДОму – stress on до-

Pronounce их and им clearly as standalone words; they are short but important for meaning.