पानी की बोतल लॉकर में है।

A natural breakdown is:

• पानी = water
• की = a linker showing possession/association here
• बोतल = bottle
• लॉकर में = in the locker
• है = is

So the structure is:

• [noun phrase] + [place phrase] + है

In other words:

• पानी की बोतल = water bottle / bottle of water
• लॉकर में = in the locker
• है = is

This is a very normal Hindi sentence pattern.

Because का / की / के agrees with the noun that comes after it, not with the possessor.

Here:

• पानी = water
• बोतल = bottle

The head noun is बोतल, and बोतल is feminine singular, so Hindi uses की:

• पानी की बोतल

Compare:

• राम का घर = Ram's house
  
  • घर is masculine singular, so का
• राम की किताब = Ram's book
  
  • किताब is feminine singular, so की
• राम के दोस्त = Ram's friends
  
  • plural, so के

So की is there because बोतल is feminine singular.

Literally, yes, it is built like water's bottle, but in real usage it means:

• water bottle
• bottle of water

Hindi often uses X का/की/के Y where English might use:

• Y of X
• X Y
• a possessive

So पानी की बोतल is just the normal Hindi way to say bottle of water or water bottle, depending on context.

Because Hindi uses postpositions, not prepositions.

English says:

• in the locker

Hindi says:

• locker in
• लॉकर में

So:

• में = in
• पर = on/at
• से = from/with/by
• को = to/object marker

These come after the noun phrase, not before it.

Hindi normally does not use articles like English a/an/the.

So पानी की बोतल लॉकर में है can mean, depending on context:

• The water bottle is in the locker
• A water bottle is in the locker

Context tells you which one is meant.

If you want to make one/a more explicit, you can sometimes use एक:

• एक पानी की बोतल लॉकर में है = A/one water bottle is in the locker

But in many normal sentences, Hindi simply leaves articles out.

Because the subject noun phrase is singular:

• बोतल = bottle = singular

So Hindi uses:

• है = is

If it were plural, you would usually use:

• हैं = are

For example:

• पानी की बोतलें लॉकर में हैं।
  = The water bottles are in the locker.

The subject is the whole noun phrase:

• पानी की बोतल

Not just पानी.

The sentence is about the bottle, not about the water by itself. So the main noun is बोतल, and that is why agreement and grammar are based on बोतल.

Yes, Hindi word order is somewhat flexible, but some orders sound more neutral than others.

The most neutral version here is:

• पानी की बोतल लॉकर में है।

You could also say:

• लॉकर में पानी की बोतल है।

This is also very natural and often sounds slightly more like There is a water bottle in the locker.

So both are correct, but they may differ slightly in emphasis:

• पानी की बोतल लॉकर में है = the bottle is in the locker
• लॉकर में पानी की बोतल है = in the locker, there is a bottle of water

It is a borrowed word, ultimately from English locker, but it is commonly used in Hindi.

Hindi uses many borrowed nouns, especially for modern objects. They behave like Hindi nouns in sentences, so you can say:

• लॉकर में = in the locker
• लॉकर का = of the locker
• लॉकर से = from the locker

So even though the word is borrowed, the grammar around it is fully Hindi.

You usually learn noun gender as part of the vocabulary item.

There is not always a perfect rule, so it is best to memorize:

• बोतल = feminine

That is why you get:

• पानी की बोतल
• not पानी का बोतल

Gender in Hindi affects words like:

• का / की / के
• adjectives in some cases
• some verb forms in certain constructions

So learning noun gender early is very helpful.

Not exactly.

• पानी की बोतल लॉकर में है = The water bottle is in the locker.
• बोतल में पानी है = There is water in the bottle.

These are different sentences with different focus:

• पानी की बोतल describes what kind of bottle it is
• बोतल में पानी tells you what is inside the bottle

So they are related, but not interchangeable.

You add नहीं before है:

• पानी की बोतल लॉकर में नहीं है।
• The water bottle is not in the locker.

This is the usual pattern:

• X Y में है = X is in Y
• X Y में नहीं है = X is not in Y

A simple yes/no question is:

• क्या पानी की बोतल लॉकर में है?
• Is the water bottle in the locker?

You can also often ask with rising intonation in speech, but क्या is the standard clear question marker at the beginning.

A wh-question could be:

• पानी की बोतल कहाँ है?
• Where is the water bottle?

A simple pronunciation guide is:

• pānī kī botal lŏkar meṅ hai

Very roughly for an English speaker:

• पानी ≈ paa-nee
• की ≈ kee
• बोतल ≈ bo-tul
• लॉकर ≈ law-kur
• में ≈ mayn with a nasal sound
• है ≈ hai like heh/hai depending on accent

The nasal sound in में is important. It is not just मे.

Because को is not needed here.

This sentence is a location sentence:

• X Y में है = X is in Y

There is no direct object here. The bottle is simply being located.

You often use को for:

• people/specific direct objects
• indirect objects
• experiencer constructions

But with simple location, Hindi normally uses:

• place + में
• then है/हैं

So:

• लॉकर में है is exactly what you would expect.