Instalatorul etanșează racordul flexibil.

Because -ul is the definite article attached to the end of a masculine/neuter singular noun in Romanian.

• instalator = a plumber
• instalatorul = the plumber
• racord = a connector / fitting
• racordul = the connector / the fitting

Unlike English, Romanian usually puts the at the end of the noun instead of using a separate word.

In Romanian, when a noun is definite and an adjective comes after it, the adjective usually does not take the definite article.

So:

• racord flexibil = a flexible connector
• racordul flexibil = the flexible connector

This is normal Romanian structure: definite noun + adjective.

Compare:

• băiat bun = a good boy
• băiatul bun = the good boy

Etanșează is the 3rd person singular present form of the verb a etanșa.

It means something like:

• seals
• makes airtight
• makes watertight
• tightens/seals properly, depending on context

In plumbing or technical contexts, a etanșa often means to seal a joint so that water, gas, or air cannot leak.

So in this sentence, it means the plumber is sealing the flexible connector/fitting.

Because a etanșa belongs to a very common Romanian verb pattern where the 3rd person singular present ends in -ează.

Here is the pattern:

• a etanșa = to seal
• el/ea etanșează = he/she seals

Other verbs work similarly:

• a lucra → lucrează = works
• a conta → contează = matters
• a forma → formează = forms

So etanșează means he/she/it seals.

The verb form helps identify the subject.

In etanșează, the ending shows it is 3rd person singular, so it can mean:

• he seals
• she seals
• it seals

Since the sentence includes instalatorul, that noun is the subject:

Instalatorul etanșează... = The plumber seals...

Romanian often does not need pronouns like he or she because the verb form already gives that information.

Because Romanian usually uses a postposed definite article—that is, the article is attached to the end of the noun.

So instead of:

• English: the plumber
• Romanian: instalatorul

And instead of:

• English: the connector
• Romanian: racordul

This is one of the most important structural differences between Romanian and English.

Not exactly. Racord is a technical word and often means:

• connector
• fitting
• joint
• coupling

In plumbing, it refers more to the connecting piece than to a full hose by itself.

So racordul flexibil is often something like:

• the flexible connector
• the flexible fitting
• sometimes the flexible hose connection, depending on context

The exact English translation can vary by the plumbing situation.

Because flexibil agrees with racord, and here the noun is singular.

Romanian adjectives agree with the noun in gender and number.

Racord is a neuter noun. In Romanian, neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So in the singular, the adjective takes the same form as masculine singular:

• racord flexibil = flexible connector

In the plural, it would change:

• racorduri flexibile = flexible connectors
• racordurile flexibile = the flexible connectors

Because pe is often used to mark a direct object when it is a person or another clearly animate/specific being.

Examples:

• Văd băiatul.
• Îl văd pe băiat.

But racordul is an inanimate object, so pe is not used here.

That is why the sentence is simply:

Instalatorul etanșează racordul flexibil.

not

Instalatorul etanșează pe racordul flexibil.

The normal, neutral order is:

subject + verb + object
Instalatorul etanșează racordul flexibil.

Romanian is somewhat more flexible than English, so other orders are possible for emphasis, but this sentence is the most natural neutral version.

For example, changing the order could sound like emphasis or contrast:

• Racordul flexibil îl etanșează instalatorul.
  = It’s the flexible connector that the plumber seals.

But for ordinary statement style, the original order is best.

Two important Romanian letters appear here:

• ș = the sh sound in ship
• ă = a short central vowel, something like the a in sofa

So:

• etanșează has ș = sh
• the final -ză ends with that Romanian ă sound
• instalatorul also ends with a reduced vowel sound in -rul

A rough pronunciation guide:

• Instalatorul ≈ in-sta-la-TOR-ul
• etanșează ≈ e-tan-SHEA-zə
• racordul ≈ ra-KOR-dul
• flexibil ≈ flek-si-BIL

The stressed syllables are usually:

• instalaTOrul
• etanȘEAză
• raCORdul
• flexiBIL

It is the direct object, so functionally it is in the accusative.

However, for most inanimate nouns in Romanian, the nominative and accusative forms are identical. So:

• racordul can be nominative: the connector as subject
• racordul can be accusative: the connector as object

In this sentence, it is accusative because it receives the action of the verb:

The plumber seals the flexible connector.

Yes. That would mean:

A plumber seals a flexible connector.

Here is the contrast:

• Instalatorul etanșează racordul flexibil. = The plumber seals the flexible connector.
• Un instalator etanșează un racord flexibil. = A plumber seals a flexible connector.

So the original sentence is definite and refers to specific things already known in context.