उस बच्ची के बाल लंबे हैं, लेकिन उसका चेहरा छोटा है।

Because उस is the oblique form of वह.

In Hindi, when a noun or pronoun comes before a postposition such as का / के / की, में, से, etc., it usually changes to its oblique form.

Here, बच्ची is followed by के in उस बच्ची के बाल, so वह becomes उस.

• direct form: वह बच्ची = that girl
• oblique form: उस बच्ची के... = of that girl / that girl’s ...

Here के shows possession, like ’s or of in English.

So:

• उस बच्ची के बाल = that girl’s hair

The basic possessive set is:

• का for masculine singular
• के for masculine plural
• की for feminine singular/plural

In this sentence, बाल is treated as masculine plural, so के is used.

These are two different but very normal ways to show possession in Hindi.

1. noun + का/के/की

   • उस बच्ची के बाल = the girl’s hair

2. pronoun possessive form

   • उसका चेहरा = her face

So the first clause uses the full noun phrase that girl, while the second switches to the pronoun her to avoid repetition.

You could also say:

• उस बच्ची का चेहरा छोटा है

That would also be correct.

Because उसका agrees with चेहरा, not with बच्ची.

This is a very common point for English speakers. In Hindi, possessive words like मेरा, तेरा, उसका, हमारा agree with the thing possessed, not with the possessor.

Here:

• possessor: the girl
• thing possessed: चेहरा = face

Since चेहरा is masculine singular, the correct form is उसका.

Compare:

• उसका चेहरा = her face
• उसकी आँखें = her eyes
• उसके बाल = her hair

Because लंबे agrees with बाल, and बाल is masculine plural here.

Hindi adjectives often change form to match the noun they describe.

• लंबा = masculine singular
• लंबे = masculine plural
• लंबी = feminine

So:

• लंबा बाल would not fit here
• लंबे बाल = long hair / long hairs

Even though English often treats hair as uncountable, Hindi usually uses बाल in the plural when talking about someone’s hair generally.

Because चेहरा is masculine singular.

The adjective must agree with चेहरा, not with बच्ची.

So:

• छोटा चेहरा = small face
• छोटी बच्ची = small/little girl

This can feel strange at first, because the person is female, but the noun face is grammatically masculine.

Because the subjects are different:

• बाल लंबे हैं → बाल is plural, so हैं
• चेहरा छोटा है → चेहरा is singular, so है

So the verb agrees with the noun being described:

• plural noun → हैं
• singular noun → है

Here बाल is functioning as a plural noun.

In Hindi, when talking about the hair on someone’s head, people very often use बाल in the plural:

• उसके बाल लंबे हैं = her hair is long / she has long hair

A single strand of hair can also be बाल, but in this sentence it clearly means hair in the general sense, and Hindi treats that as plural.

That is why you get:

• के
• लंबे
• हैं

all in plural agreement.

It can mean either his or her.

Hindi third-person pronouns do not usually change for natural gender the way English does. So:

• उसका can mean his, her, or sometimes its, depending on context.

In this sentence, the context is बच्ची (girl), so उसका is understood as her.

लेकिन means but.

It connects two contrasting statements:

• उस बच्ची के बाल लंबे हैं
• उसका चेहरा छोटा है

So the whole sentence means something like:

• her hair is long, but her face is small

Placing लेकिन between the two clauses is very natural in Hindi.

You may also hear:

• मगर
• पर

They can often mean the same thing as लेकिन in everyday speech.

Yes. A more fully parallel version would be:

• उस बच्ची के बाल लंबे हैं, लेकिन उस बच्ची का चेहरा छोटा है।

Or more naturally:

• उस बच्ची के बाल लंबे हैं, लेकिन उसका चेहरा छोटा है।

The original version sounds more natural because Hindi, like English, often avoids repeating the noun when the reference is already clear.

The structure is:

• [possessor phrase] + [noun] + [adjective] + [copula]

So:

• उस बच्ची के बाल लंबे हैं

  • that girl’s hair long is/are

• उसका चेहरा छोटा है

  • her face small is

Hindi often puts the descriptive adjective before है / हैं, while English uses is/are before the adjective:

• Hindi: बाल लंबे हैं
• English: hair is long

That word order difference is very normal.