Водопроводная труба находится под раковиной.

Because под can take two different cases in Russian:

• Instrumental when it means location: under / beneath
• Accusative when it means motion toward a position under something: to under / underneath

In this sentence, the pipe is already located under the sink, so Russian uses the instrumental:

• под раковиной = under the sink

Compare:

• Труба находится под раковиной. = The pipe is under the sink.
• Положи трубу под раковину. = Put the pipe under the sink.

So:

• под раковиной = location
• под раковину = movement/direction

Because раковина is a feminine noun, and after под in the meaning of location, it must be in the instrumental singular.

The forms are:

• nominative: раковина
• instrumental: раковиной

This is a very common feminine singular instrumental ending:

• -а → -ой / -ей
• раковина → раковиной
• лампа → лампой
• дверь → дверью (different pattern, because it is a different noun type)

So раковиной is simply the grammatically required form after под here.

Труба by itself can mean many kinds of pipe or tube, and even trumpet in some contexts.
The adjective водопроводная specifies that it is a water pipe / plumbing pipe.

So:

• труба = pipe, tube
• водопроводная труба = water pipe, plumbing pipe

The adjective comes from водопровод = water supply system / plumbing / water mains.

Russian often uses adjective + noun combinations like this to be more specific.

Because it has to agree with the noun труба, which is feminine singular.

In Russian, adjectives agree with the noun in:

• gender
• number
• case

Here:

• труба = feminine, singular, nominative
• so the adjective must also be feminine, singular, nominative: водопроводная

Compare:

• водопроводная труба = water pipe
• водопроводный кран = water tap (masculine noun)
• водопроводное отверстие = water opening (neuter noun)
• водопроводные трубы = water pipes (plural)

Yes, in some contexts you could omit it, but находится makes the sentence feel more complete and explicit.

Находиться means to be located, to be situated.

So:

• Водопроводная труба находится под раковиной. = The water pipe is located under the sink.

Without it:

• Водопроводная труба под раковиной.

This can still be understood, especially in conversation, but it may sound more like a shortened statement, label, or answer depending on context.

Russian often omits to be in the present tense, but verbs like находиться, лежать, стоять, висеть can be added when you want to describe location more precisely.

Russian has no present-tense form of "to be" like English is in ordinary sentences.

So instead of saying a direct equivalent of is, Russian often either:

• leaves it out completely, or
• uses a more specific verb

Here, находится means something like:

• is located
• is situated

So the sentence is a little more formal or descriptive than a simple English is.

Compare:

• Книга на столе. = The book is on the table.
• Книга находится на столе. = The book is located on the table.

Both are correct, but находится sounds more explicit and sometimes slightly more formal.

Yes, Russian word order is flexible. This sentence uses a very neutral order:

• subject: Водопроводная труба
• verb: находится
• location: под раковиной

This is the most straightforward way to say it.

But other orders are also possible, depending on emphasis:

• Под раковиной находится водопроводная труба.
  Emphasizes the location: Under the sink, there is a water pipe.

• Находится водопроводная труба под раковиной.
  Possible, but less neutral and more marked.

For learners, the original order is the best default pattern to copy.

In this sentence, yes, раковина most naturally means sink.

The word раковина can also mean:

• seashell / shell
• sink / washbasin

But the phrase под раковиной together with водопроводная труба makes the meaning completely clear: it means a sink.

This kind of ambiguity is normal in Russian, just as in English some words have multiple meanings.

The stress is:

• водопрово́дная
• труба́
• нахо́дится
• под ракови́ной

A rough pronunciation guide:

• водопрово́дная = va-da-pra-VOD-na-ya
• труба́ = troo-BA
• нахо́дится = na-HOD-it-sa
• под ракови́ной = pot ra-ka-VEE-noy

A few useful pronunciation notes:

• Unstressed о often sounds closer to a.
• Final д in под is pronounced more like т before a voiceless consonant.
• -тся in находится is pronounced roughly -tsa.

Russian has no articles like a/an/the.

So:

• труба can mean a pipe or the pipe
• раковина can mean a sink or the sink

The exact meaning depends on context.

In this sentence, English naturally uses the because the listener can imagine a specific sink and a specific pipe:

• The water pipe is under the sink.

But Russian does not need a separate word for that.

Yes. In many contexts, под can be translated as:

• under
• below
• beneath

In this sentence, under is the most natural English choice:

• под раковиной = under the sink

But the core spatial meaning is the same.

Yes, it is a natural and correct phrase.

Depending on context, Russian speakers might also use related expressions such as:

• труба водопровода = pipe of the water supply system
• водяная труба = water pipe
• труба под раковиной = the pipe under the sink

But водопроводная труба is a standard, clear way to say water pipe or plumbing pipe.

It is in the nominative singular because it is the subject of the sentence.

You can see this in both words:

• водопроводная = feminine singular nominative
• труба = feminine singular nominative

The sentence structure is:

• Водопроводная труба — subject
• находится — verb
• под раковиной — prepositional phrase showing location

So the subject stays in the nominative, while the noun after под changes to instrumental.