Fachowiec wymienia starą rurę, bo pod zlewem jest przeciek.

Fachowiec means a skilled worker, specialist, tradesperson, or professional repair person. It is fairly general.

In this sentence, the context suggests someone doing repair work, and because the problem is a pipe under a sink, an English speaker would often naturally think of a plumber. But Polish fachowiec itself does not specifically mean plumber.

If you wanted to be specific, you could say hydraulik for plumber.

Wymienia is 3rd person singular present tense of wymieniać.

So literally it means replaces / is replacing.

In Polish, the present tense of an imperfective verb can describe:

• something happening right now: is replacing
• something done regularly / generally: replaces

Here, the most natural English translation is usually is replacing, because it sounds like an action in progress.

This is about aspect, which is very important in Polish verbs.

• wymieniać = imperfective
• wymienić = perfective

In this sentence, wymienia comes from the imperfective verb wymieniać, which is good for describing an action as ongoing or in progress.

If you used the perfective verb:

• wymieni would usually mean will replace (future), not is replacing now

So:

• Fachowiec wymienia starą rurę = The repairman is replacing the old pipe
• Fachowiec wymieni starą rurę = The repairman will replace the old pipe

Because rura is the direct object of the verb wymienia.

The verb wymieniać takes the accusative case, so both the noun and the adjective must change form.

Base form:

• stara rura = old pipe nominative

In the sentence:

• starą rurę = accusative singular

Why those endings?

• rura is a feminine noun
• feminine singular accusative usually changes:
  • -a → -ę for the noun
  • -a → -ą for the adjective

So:

• stara rura → starą rurę

Zlewem is in the instrumental case.

That happens because pod can take different cases depending on the meaning:

• pod + instrumental = location: under / beneath
• pod + accusative = movement toward a position under something

Here we have a static location:

• pod zlewem jest przeciek = there is a leak under the sink

So we use instrumental:

• zlew → zlewem

Compare:

• Klucz leży pod zlewem. = The key is lying under the sink.
• Wkładam pudełko pod zlew. = I am putting the box under the sink.

In the second sentence, there is motion, so pod takes the accusative.

Przeciek is a noun meaning leak.

So:

• jest przeciek = there is a leak

It is not a verb. The verb would be something like:

• przeciekać = to leak

So compare:

• Pod zlewem jest przeciek. = There is a leak under the sink.
• Rura przecieka. = The pipe is leaking.

English often prefers the verbal expression, but Polish can use either a noun or a verb depending on style and focus.

Because Polish does not need a dummy subject like English there.

English says:

• There is a leak under the sink

Polish simply says:

• Pod zlewem jest przeciek
• literally: Under the sink is a leak

This is normal in Polish. There is no need to add a separate word meaning English grammatical there.

Also, Polish has no articles, so:

• jest przeciek can mean there is a leak
• no word for a is needed

Because bo means because, and in standard Polish punctuation it is normally preceded by a comma when it introduces a clause.

So:

• Fachowiec wymienia starą rurę, bo pod zlewem jest przeciek.

This is the standard written form.

A useful rule for learners: when bo links two clauses, you usually put a comma before it.

No, Polish word order is fairly flexible, because case endings show the grammatical roles.

The sentence given is neutral and natural:

• Fachowiec wymienia starą rurę, bo pod zlewem jest przeciek.

But other orders are possible if you want different emphasis, for example:

• Starą rurę wymienia fachowiec...
  This puts more focus on the old pipe
• Bo pod zlewem jest przeciek, fachowiec wymienia starą rurę.
  This puts the reason first

That said, not every possible order sounds equally natural in every context. The version you were given is a good standard one.

Yes, if you want to be more specific.

• fachowiec = skilled worker / repair professional / tradesperson
• hydraulik = plumber

So:

• Hydraulik wymienia starą rurę, bo pod zlewem jest przeciek.

This would mean specifically:

• The plumber is replacing the old pipe because there is a leak under the sink.

Using fachowiec is broader and slightly less specific.