Сантехник решил заменить прокладку.

Because it is the direct object of the verb заменить (to replace), so it goes into the accusative case.

• Dictionary form: прокладка
• In this sentence: прокладку

This happens because прокладка is a feminine noun ending in -а. For many feminine nouns like this, the accusative singular changes -а to -у:

• книга → книгу
• машина → машину
• прокладка → прокладку

So заменить прокладку means to replace a gasket/washer/seal.

Заменить is the perfective form, which focuses on the action as a completed whole: replacing it successfully.

Russian often uses the perfective infinitive after решить when someone decides to do something конкретely and to completion:

• решил заменить = decided to replace
• решил купить = decided to buy
• решил уйти = decided to leave

By contrast:

• менять usually means to change / be changing
• заменять is the imperfective partner of заменить, often used for repeated, ongoing, or general replacement

So решил заменить прокладку sounds natural because the idea is: the plumber made a decision to carry out one complete replacement.

This is a very common Russian pattern.

• решил = decided
• заменить = to replace

After verbs like решить, хотеть, мочь, любить, Russian often uses an infinitive:

• решил сделать = decided to do
• хочет купить = wants to buy
• может прийти = can come

So решил заменить works just like English decided to replace.

Because the subject, сантехник, is a masculine noun.

In the past tense, Russian verbs agree in gender and number with the subject:

• masculine: решил
• feminine: решила
• neuter: решило
• plural: решили

Since сантехник is masculine singular, the sentence uses решил.

Compare:

• Сантехник решил заменить прокладку.
• Сантехникa? No, here сантехник is the subject, so nominative is used.
• Сантехница решила заменить прокладку. if the subject were feminine

Yes. Сантехник is the subject of the sentence, so it is in the nominative case.

You can tell because it is the person doing the action:

• Сантехник decided
• Сантехник replaced / was going to replace

In basic Russian sentences, the subject is usually nominative, while the direct object often takes the accusative.

So here:

• Сантехник = nominative subject
• прокладку = accusative object

Not always. Прокладка is a broad word whose exact meaning depends on context.

In plumbing, it often means something like:

• gasket
• washer
• seal

In other contexts, it can mean other kinds of layers, inserts, or padding. So in this sentence, because the subject is сантехник (plumber), the plumbing meaning is the natural one.

The stress is:

• сантЕхник
• решИл
• заменИть
• проклАдку

Stress is important in Russian because it is not always predictable and can change between forms.

A rough pronunciation guide:

• сантЕхник ≈ san-TEKH-neek
• решИл ≈ ree-SHEEL
• заменИть ≈ za-me-NEET’
• проклАдку ≈ pra-KLAHD-koo

Yes. Russian word order is more flexible than English because case endings show grammatical roles.

The neutral word order here is:

• Сантехник решил заменить прокладку.

But you could also hear:

• Прокладку сантехник решил заменить.
• Решил сантехник заменить прокладку. (more marked, depending on context)

Changing the word order usually changes the focus or emphasis, not the basic meaning.

For a learner, the given order is the most straightforward and natural.

Russian has no articles like English a/an/the.

So сантехник can mean:

• a plumber
• the plumber

And прокладку can mean:

• a gasket
• the gasket

Which one is meant depends on the context, not on a separate word.

Решил is the past tense of решить, which is usually treated as perfective in this meaning: to decide.

So the sentence has two perfective verbs:

• решил = decided
• заменить = to replace completely

This gives the sentence a clear, goal-oriented feel: he came to a decision, and the intended action is a complete replacement.

Yes, but it means something slightly different.

• решил заменить = decided to replace
• хотел заменить = wanted to replace

Решил is stronger: it suggests the person made a definite decision.

Хотел only tells you about the desire or intention, not necessarily a firm decision.

It is usually followed directly by the object in the accusative case, with no preposition:

• заменить лампочку
• заменить дверь
• заменить прокладку

So заменить прокладку is the normal pattern.

If you want to mention what it is replaced with, then Russian can use another construction, for example with на:

• заменить старую прокладку на новую = replace the old gasket with a new one / replace the old gasket for a new one

But in your sentence, only the thing being replaced is mentioned.