В оголошенні було написано, що меблі нові, але диван виявився старим.

Because after в / у meaning in, Ukrainian uses the locative case for location.

• Dictionary form: оголошення = announcement, ad
• Locative singular: в оголошенні = in the announcement / in the ad

So:

• в оголошенні = in the ad
• в оголошення would be ungrammatical here

The ending -і is a common locative ending for neuter nouns like оголошення.

Було написано is an impersonal passive construction. It means something like it was written.

Breakdown:

• було = was
• написано = written

This structure is very common in Ukrainian when the speaker cares about the result or content, not about who did the writing.

So:

• В оголошенні було написано... = In the ad, it said... / It was written in the ad...

A native English speaker may expect something more literal like the ad was written, but that would mean something different. Here the sentence is talking about the content inside the ad, not the act of composing the ad.

Here що means that and introduces a subordinate clause.

So:

• було написано, що меблі нові = it was written that the furniture was new

The comma is required before що because Ukrainian normally separates this kind of subordinate clause with a comma.

This is very similar to English:

• It said that the furniture was new.

English sometimes omits that, but Ukrainian usually keeps що.

In Ukrainian, меблі is a plural-only noun. That means it looks grammatically plural even though in English the equivalent word furniture behaves as singular and uncountable.

So Ukrainian says:

• меблі нові = literally the furniture [are] new

This is normal Ukrainian grammar. You should treat меблі as a plural noun whenever you use adjectives or verbs with it.

For example:

• Ці меблі дорогі. = This furniture is expensive.
• Меблі були старі. = The furniture was old.

Because нові is the plural form of the adjective новий (new), and it must agree with меблі, which is grammatically plural.

Agreement here is:

• меблі = plural
• нові = plural adjective

So:

• меблі нові = the furniture is new

Even though English uses singular furniture, Ukrainian grammar requires the plural adjective.

Виявився means turned out to be or proved to be. It suggests that the real situation was discovered later and was different from what was expected.

So:

• диван був старим = the sofa was old
• диван виявився старим = the sofa turned out to be old

The second version is more natural here because the sentence contrasts the claim in the ad with the reality.

That contrast is important:

• the ad said one thing
• reality showed something else

So виявився adds the idea of unexpected discovery.

Yes, -ся is the reflexive marker, but in many Ukrainian verbs it does not mean literal self-action like himself or itself. Sometimes it is simply part of the verb’s normal dictionary form.

Here the verb is:

• виявитися = to turn out, to prove to be

So виявився is just the past masculine singular form of that verb.

You do not need to interpret it literally as revealed itself in this sentence. The important thing is to learn виявитися as a whole verb meaning to turn out.

Because after виявитися Ukrainian normally uses the instrumental case for the predicate noun or adjective.

So:

• диван виявився старим = the sofa turned out to be old

Here:

• старий = old, masculine nominative
• старим = old, masculine instrumental

This is a very common pattern after verbs like:

• бути in some structures
• стати = become
• виявитися = turn out to be
• залишитися = remain

Compare:

• Він став лікарем. = He became a doctor.
• Вона виявилася правою. = She turned out to be right.
• Диван виявився старим. = The sofa turned out to be old.

So the instrumental here is not random; it is required by the construction.

Because it agrees with диван, which is:

• masculine
• singular

In the past tense, Ukrainian verbs usually show gender in the singular:

• виявився = masculine singular
• виявилася = feminine singular
• виявилося = neuter singular
• виявилися = plural

So if the subject changed, the form would change too:

• Лампа виявилася старою. = The lamp turned out to be old.
• Крісло виявилося старим. = The armchair turned out to be old.
• Меблі виявилися старими. = The furniture turned out to be old.

Because але means but, and Ukrainian normally puts a comma before coordinating conjunctions like this when they connect two clauses.

So the sentence has two parts:

1. В оголошенні було написано, що меблі нові
2. але диван виявився старим

The comma marks the contrast between them.

This is similar to English punctuation in a sentence like:

• The ad said the furniture was new, but the sofa turned out to be old.

Yes, Ukrainian word order is fairly flexible, although the original version is very natural.

Original:

• В оголошенні було написано, що меблі нові, але диван виявився старим.

You could also say:

• Було написано в оголошенні, що меблі нові, але диван виявився старим.

But that sounds a bit less neutral.

Or:

• Але диван виявився старим could be moved for emphasis in a larger context.

In general, the given word order is good because it presents the information clearly:

1. what the ad claimed
2. what reality turned out to be

So yes, word order can change, but not every version sounds equally natural.

Because Ukrainian has no articles. It does not have direct equivalents of English a/an and the.

So:

• в оголошенні can mean in the ad or in an ad
• диван can mean the sofa or a sofa

The exact meaning depends on context.

In this sentence, context makes it clear that English would naturally say the ad and the sofa, but Ukrainian does not need separate words for that.

Yes. В and у often alternate in Ukrainian for ease of pronunciation.

So both are possible:

• в оголошенні
• у оголошенні

Speakers choose the version that sounds smoother next to surrounding words. This is a phonetic choice, not a change in meaning.

In your sentence, В оголошенні is perfectly natural.