Их племянник иногда помогает им по дому.

Their nephew sometimes helps them around the house.

Breakdown of Их племянник иногда помогает им по дому.

• дом: the house
• помогать: to help
• по: around
• иногда: sometimes

Questions & Answers about Их племянник иногда помогает им по дому.

Why is их used here, and what does it mean?

Их means their. It is the possessive form related to они (they).
Two important points:

1. 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

2. 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)?