बिल्ली की हरकत तेज़ है।

की is the genitive marker here, roughly like of or the English 's.

So:

• बिल्ली की हरकत = the cat's movement / action / behavior

A very important point: का / की / के agree with the noun that comes after them, not with the owner.

Here the following noun is हरकत, which is feminine singular, so Hindi uses की.

For comparison:

• बिल्ली का बच्चा = the cat's kitten
  
  • बच्चा is masculine singular, so का
• बिल्ली की हरकत = the cat's movement
  
  • हरकत is feminine singular, so की

It agrees with हरकत, not with बिल्ली.

This is one of the most common things English speakers need to get used to in Hindi. In English, we do not change 's depending on the noun after it, but in Hindi the possessive marker changes form:

• का for masculine singular
• की for feminine
• के for masculine plural and some other contexts

So in बिल्ली की हरकत, the key reason for की is that हरकत is feminine.

हरकत can mean movement, motion, action, gesture, or even behavior/antics, depending on context.

In this sentence, it most likely refers to the cat's movement or way of moving.

But learners should know that हरकत is a bit broader than a simple physical-motion word. In some contexts it can sound like:

• an action someone did
• a gesture
• behavior, sometimes even slightly disapproving, like antics

So the exact nuance depends on the situation.

Here तेज़ means fast or quick.

This word is quite flexible in Hindi and can also mean things like:

• sharp
• strong
• intense
• bright
• loud, in some contexts

But with हरकत in this sentence, the natural meaning is fast or quick.

Because the main noun being described is हरकत, and हरकत is singular.

So:

• हरकत ... है = the movement is ...
• हरकतें ... हैं = the movements are ...

In this sentence:

• बिल्ली की हरकत = singular noun phrase
• therefore तेज़ है

If it were plural, you could have:

• बिल्ली की हरकतें तेज़ हैं।
  = The cat's movements are fast.

No, तेज़ stays the same here.

Many Hindi adjectives do change, especially ones like:

• अच्छा / अच्छी / अच्छे
• बड़ा / बड़ी / बड़े

But तेज़ is an indeclinable adjective in normal use, so it usually does not change form for masculine/feminine or singular/plural.

So you can say:

• तेज़ बिल्ली
• तेज़ कुत्ता
• तेज़ हरकत
• तेज़ हरकतें

The adjective तेज़ remains तेज़.

The sentence is:

• बिल्ली की हरकत = the cat's movement/action
• तेज़ = fast
• है = is

So the structure is:

[noun phrase] + [predicate adjective] + [copula]

That is a very common Hindi pattern:

• लड़का लंबा है। = The boy is tall.
• गाड़ी तेज़ है। = The car is fast.
• बिल्ली की हरकत तेज़ है। = The cat's movement is fast.

Yes, and that is often more natural if you simply mean The cat is fast.

The difference is:

• बिल्ली तेज़ है। = The cat is fast.
• बिल्ली की हरकत तेज़ है। = The cat's movement/action is fast.

The second sentence focuses specifically on the cat's movement or behavior, not just the cat as a whole.

So if you are talking about speed in a general way, बिल्ली तेज़ है may be simpler and more natural.

It is grammatically correct, but the naturalness depends on what exactly you want to say.

If you mean:

• the cat moves quickly
• the cat is agile
• the cat is fast

then native speakers might often say something more natural such as:

• बिल्ली तेज़ है।
• बिल्ली बहुत फुर्तीली है। = The cat is very agile.
• बिल्ली की चाल तेज़ है। = The cat's gait/movement is fast.

हरकत is not wrong, but it can sound a little broader, like movement, motion, or even behavior/antics, rather than just straightforward speed.

Hindi normally does not use articles like a and the.

So बिल्ली can mean:

• a cat
• the cat

The exact meaning comes from context.

That is why Hindi learners should not expect a separate word corresponding directly to English the in most sentences.

तेज़ is usually pronounced roughly like tez.

The letter ज़ represents a z sound. The dot under the letter shows a sound borrowed into Hindi from Persian/Arabic sources.

So:

• तेज would more traditionally suggest a j-type sound in Sanskrit-based spelling
• तेज़ specifically marks the z pronunciation

In everyday speech, pronunciation can vary a bit by speaker and region, but tez is the standard learning pronunciation here.

A simple learner-friendly pronunciation is:

billī kī harkat tez hai

Very roughly in English-style sounds:

• बिल्ली = bill-ee
• की = kee
• हरकत = hur-kut or har-kut
• तेज़ = tez
• है = hai, often sounding a bit like heh or hai depending on speech

So the whole sentence is:

billī kī harkat tez hai