यह बस ट्रेन से सस्ती है, लेकिन ट्रेन उससे तेज़ है।

Because बस is a feminine singular noun in Hindi, and सस्ता is a changeable adjective that must agree with it.

The adjective changes like this:

• सस्ता = masculine singular
• सस्ती = feminine singular
• सस्ते = masculine plural / some oblique uses

So:

• बस सस्ती है = The bus is cheap.
• ट्रेन सस्ती है = The train is cheap.

In this sentence, सस्ती describes बस, so it must be feminine.

Here, से means than in a comparison.

So:

• ट्रेन से सस्ती = cheaper than the train
• उससे तेज़ = faster than that / than it

This is a very common Hindi pattern:

• X Y से बड़ा है = X is bigger than Y
• X Y से अच्छा है = X is better than Y
• X Y से महँगा है = X is more expensive than Y

Even though से can also mean from, with, or by, in this sentence it is specifically the comparison marker.

Hindi and English organize comparisons differently.

Hindi commonly uses this pattern:

X + Y से + adjective + है

So:

• बस ट्रेन से सस्ती है = Bus train-than cheap is = The bus is cheaper than the train

English uses:

X + is + adjective + than + Y

So the meaning is the same, but the structure is different. This word order is completely normal in Hindi.

उससे is made of:

• उस = oblique form of वह (that / it / he / she)
• से = than

So उससे means than that or more naturally here, than it.

In this sentence, उससे refers back to this bus. So:

• ट्रेन उससे तेज़ है = The train is faster than it = The train is faster than the bus

Hindi often uses a pronoun like this instead of repeating the noun.

Because Hindi pronouns change form before postpositions like से, को, में, पर, etc.

This is called the oblique form.

For वह, the forms are:

• direct: वह
• oblique: उस

So:

• वह = that / he / she / it
• उससे = from that / than that / with that

The same kind of change happens with other pronouns too:

• यह → इस
• वे → उन
• ये → इन

So उससे is grammatically required here.

Because तेज़ is an indeclinable adjective in modern Hindi. That means it usually does not change form for gender or number.

So you can say:

• बस तेज़ है
• ट्रेन तेज़ है
• लड़का तेज़ है
• लड़कियाँ तेज़ हैं

By contrast, सस्ता is a declinable adjective, so it changes:

• सस्ता लड़का
• सस्ती बस
• सस्ते लड़के

So the difference is not about the sentence structure; it is about the type of adjective.

Yes. You could say:

यह bus ट्रेन से सस्ती है, लेकिन ट्रेन बस से तेज़ है।

That is grammatical and clear.

But उससे is often more natural because it avoids repetition:

• ट्रेन उससे तेज़ है = the train is faster than that / it

Hindi, like English, often uses pronouns when the reference is already clear.

Because each part is its own clause, and each clause needs the verb to be in standard Hindi.

The sentence has two linked statements:

1. यह बस ट्रेन से sस्ती है
2. लेकिन ट्रेन उससे तेज़ है

Each one ends with है because each one says what something is.

In casual speech, people may sometimes drop a final है if the meaning is obvious, but in normal careful Hindi, especially in writing, keeping both is standard.

लेकिन means but.

It connects two contrasting ideas:

• the bus is cheaper than the train
• but the train is faster than the bus

Other Hindi words with a similar meaning include:

• पर
• मगर

But लेकिन is a very common and neutral choice.

यह means this, so यह बस means this bus.

It points out a specific bus, usually one that is near the speaker or already being discussed.

So:

• यह बस = this bus
• वह बस = that bus

A learner may also notice that in everyday speech, many speakers say ये for singular this as well, but यह is the standard written form.