जब मेरी बहन उदास होती है, तब वह कमरे में बैठती है।

जब ... तब is a common Hindi correlative pair meaning when ... then or whenever ... then.

So:

• जब मेरी बहन udās hotī hai = when/whenever my sister is sad
• तब वह कमरे में बैठती है = then she sits in the room

In English, we often do not say then, but Hindi often uses तब to neatly match जब.

You can also very often omit तब in natural Hindi:

• जब मेरी बहन उदास होती है, वह कमरे में बैठती है।

That still sounds correct.

Here होती है comes from the verb होना (to be / to become / to happen, depending on context).

In this sentence, उदास होती है means is sad in a habitual or repeated sense.

So it is not talking about one exact moment only. It means something like:

• When my sister is sad
• or more naturally, Whenever my sister feels sad

Because the subject बहन is feminine singular, the verb form is also feminine singular:

• masculine singular: होता है
• feminine singular: होती है

बैठती है is the habitual present form of बैठना (to sit).

It means sits or usually sits.

So the whole sentence gives a repeated or general pattern:

• Whenever my sister is sad, she sits in the room.

Because वह refers to बहन, which is feminine singular, the verb becomes बैठती है:

• masculine singular: बैठता है
• feminine singular: बैठती है

If you wanted is sitting right now, Hindi would normally use a progressive form such as:

• बैठ रही है

So:

• बैठती है = sits / usually sits
• बैठ रही है = is sitting

It normally sounds habitual or general, not like a one-time event.

So the meaning is closer to:

• Whenever my sister gets sad, she sits in her room
• When my sister is sad, she tends to sit in the room

The forms होती है and बैठती है both suggest something repeated, usual, or characteristic.

If you wanted one specific occasion, Hindi would more likely use different forms, for example:

• जब मेरी बहन उदास होती है = general/habitual
• जब मेरी बहन उदास हुई, तब वह कमरे में बैठ गई = one specific past event

Because मेरा / मेरी / मेरे agree with the thing possessed, not with the possessor.

Here the possessed noun is बहन (sister), which is feminine singular, so the correct form is:

• मेरी बहन = my sister

Compare:

• मेरा भाई = my brother
• मेरी बहन = my sister
• मेरे दोस्त = my friends / my male friend in some contexts with oblique/plural agreement patterns

So मेरी is used because बहन is feminine.

Because उदास is an indeclinable adjective in normal usage here. That means it does not change form for masculine/feminine or singular/plural.

So you can say:

• वह लड़का उदास है = that boy is sad
• वह लड़की उदास है = that girl is sad
• वे बच्चे उदास हैं = those children are sad

By contrast, some Hindi adjectives do change, especially many ending in -ा in the masculine singular form, such as:

• अच्छा लड़का
• अच्छी लड़की

But उदास stays उदास.

This happens because Hindi nouns often change to an oblique form before a postposition such as:

• में = in
• पर = on
• से = from/with
• को = to

The basic noun is:

• कमरा = room

Before में, it becomes:

• कमरे में = in the room

This is very common with many masculine nouns ending in -ा:

• लड़का → लड़के को
• कमरा → कमरे में
• किताब → किताब में (no change here because this noun follows a different pattern)

So कमरे is the oblique form of कमरा.

में is a postposition meaning in or inside here.

So:

• कमरे में = in the room

Hindi uses postpositions after nouns, where English uses prepositions before nouns.

Compare:

• English: in the room
• Hindi: room in → कमरे में

That is why you see:

• वह कमरे में बैठती है = she sits in the room

Hindi usually follows Subject-Object-Verb order, and verbs often come later than in English.

This sentence is structured roughly like this:

• जब मेरी बहन उदास होती है
  = when my sister sad is
• तब वह कमरे में बैठती है
  = then she room-in sits

That sounds strange in English, but it is normal in Hindi.

A very literal breakdown is:

• जब = when
• मेरी बहन = my sister
• उदास = sad
• होती है = is
• तब = then
• वह = she
• कमरे में = in the room
• बैठती है = sits

So the main thing to remember is: Hindi often places the verb later than English does.

Yes, very often.

Both of these are natural:

• जब मेरी बहन उदास होती है, तब वह कमरे में बैठती है।
• जब मेरी बहन उदास होती है, वह कमरे में बैठती है।

Including तब can make the sentence feel a bit more balanced or explicit: when X, then Y.

Omitting it is also common in everyday Hindi.

वह can mean:

• he
• she
• that

Hindi third-person pronouns do not mark gender the way English pronouns do.

In this sentence, वह clearly means she because it refers back to मेरी बहन.

Also, the verb बैठती है is feminine, which confirms that the subject is feminine.

In speech, many people pronounce वह more like वो, but in standard writing वह is very common.

We know from both meaning and verb agreement.

First, the sentence introduces मेरी बहन and then continues with वह, which naturally refers back to her.

Second, the verb is feminine:

• बैठती है

That matches बहन, a feminine noun.

If the subject were masculine, we would expect:

• बैठता है

So the grammar helps tell you that वह here means she.

Yes, absolutely. That is actually a very natural way to understand it.

Because the sentence uses habitual forms:

• उदास होती है
• बैठती है

it often implies whenever or whenever it happens that.

So possible natural English translations include:

• When my sister is sad, she sits in the room.
• Whenever my sister is sad, she sits in the room.
• Whenever my sister feels sad, she sits in the room.

The exact English choice depends on style, but the Hindi sentence naturally suggests a repeated situation.

Usually sits is better here.

बैठना can sometimes mean to sit down, depending on context, but in a habitual sentence like this, बैठती है most naturally means:

• sits
• goes and sits
• stays sitting in a loose everyday sense

If Hindi wanted to strongly emphasize the action of sitting down, it might use context or another form, such as:

• बैठ जाती है

But in your sentence, the safest natural translation is simply:

• she sits in the room

Because both verbs agree with the feminine singular subject, which is मेरी बहन.

So:

• मेरी बहन उदास होती है
• वह कमरे में बैठती है

Both clauses are about the same person: my sister.

That is why both verbs use the feminine singular -ती है pattern.

Compare:

• मेरा भाई उदास होता है, तब वह कमरे में बैठता है।
  = My brother is sad, then he sits in the room.

So this sentence is a good example of how Hindi verbs often show gender agreement clearly.