Накладку недавно заменили.

Because заменили does not agree with накладку.

Here, накладку is the object of the verb, not the subject. The verb is in the plural past because Russian is using an indefinite-personal construction: an omitted they.

So the sentence is literally something like:

• They replaced the pad/lining recently

But in natural English, this is often translated as:

• It was replaced recently
• The pad/lining was recently replaced

Russian uses this pattern very often when the people who did the action are unknown, unimportant, or just not being mentioned.

Russian often leaves out the subject when it is not needed.

In this sentence, the idea is someone replaced it, but Russian does not have to say who. Using a plural verb with no stated subject is a normal way to express that.

So заменили by itself can mean:

• they replaced
• someone replaced
• it got replaced (in English translation)

This is a very common Russian pattern.

Because the noun is in the accusative case, which is used here for the direct object.

The dictionary form is накладка.
Since it is a feminine noun ending in -а, its accusative singular changes to -у:

• накладка → nominative singular
• накладку → accusative singular

So in this sentence:

• Накладку = the pad/lining/etc. as the thing that was replaced

The dictionary form is накладка.

From that form, you can see that it is:

• feminine
• singular
• a typical first-declension noun

In the sentence, накладку is:

• accusative singular feminine

So this is a good example of a very common pattern:

• feminine noun in -а
• direct object in accusative singular
• ending changes to -у

Заменили is:

• past tense
• plural
• perfective aspect

Its infinitive is заменить.

The perfective aspect shows the action as completed: the replacement happened and is done.

That fits well with недавно:

• Накладку недавно заменили.
• They replaced it recently / It was replaced recently.

If you used the imperfective заменяли, the meaning would shift toward process, repetition, or background action, for example:

• Накладку заменяли несколько раз. = They replaced the pad several times.

So заменили is the natural choice for a single completed replacement.

Grammatically, it is active, not passive.

The verb form заменили is an active past-tense verb: they replaced.

However, because the subject is omitted and unimportant, English often translates this kind of sentence with a passive:

• It was replaced recently

So:

• Russian grammar: active indefinite-personal construction
• Typical English translation: often passive

That is why the Russian and English structures may look different even though the meaning is the same.

Yes, absolutely.

That version is a more straightforward passive construction:

• Накладка была недавно заменена.

This also means The pad/lining was recently replaced.

The difference is mostly stylistic:

• Накладку недавно заменили.
  More natural and common in everyday Russian; focuses on the action without naming the doer.
• Накладка была недавно заменена.
  More formal, more written, more explicitly passive.

Both are correct, but the first one is often the more idiomatic everyday choice.

Yes, Russian word order is flexible.

In Накладку недавно заменили, the object накладку comes first, which can make it feel like the item is already known in the conversation and is the main topic.

The adverb недавно before the verb is very natural.

Other possible orders include:

• Недавно заменили накладку.
  A bit more neutral in some contexts: Recently they replaced the pad.
• Накладку заменили недавно.
  This can put more emphasis on recently.

So the word order changes the focus or information structure, not the basic meaning.

No. Недавно is an adverb, so it does not decline.

It stays the same regardless of:

• case
• gender
• number

Its role is simply to describe when the action happened: recently / not long ago.

The stress is:

• наклАдку
• недАвно
• заменИли

So the whole sentence is pronounced roughly as:

• наклАдку недАвно заменИли

Stress matters in Russian, so it is useful to learn each word together with its stressed syllable.