kekkou hurui tatemono desu ga, naka ha omotta yori kirei desita.

Here, けっこう means fairly, quite, or rather.

So けっこう 古い 建物 means a საკმაოდ old building / a fairly old building.

A useful thing to know is that けっこう can have different meanings depending on context. For example, it can also mean fine, all right, or even no thank you in other situations. But in this sentence, it is simply an adverb modifying 古い.

Because 古い is an i-adjective.

In Japanese, i-adjectives directly modify nouns:

• 古い 建物 = an old building
• 新しい 車 = a new car

You do not change 古い into 古な because な is used with na-adjectives, not i-adjectives.

ですが means but or though here.

The sentence is divided into two contrasting parts:

• けっこう 古い 建物 です が
• 中 は 思った より きれい でした

So the speaker is saying something like:

• It’s a fairly old building, but...
• The inside was cleaner/prettier than I expected.

This contrast is the main job of が here.

Also, ですが is a polite form:

• です + が

In everyday speech, people might also say:

• けっこう古い建物だけど…
• けっこう古い建物だが… (less common in casual conversation)

は marks 中 as the topic.

• 中 = inside / interior
• 中は = as for the inside

This creates a contrast between the building itself and its interior:

• The building is fairly old,
• but as for the inside, it was cleaner than expected.

So は helps highlight the inside as the thing being commented on.

In this sentence, 中 means the inside or the interior of the building.

So:

• 建物 = building
• 中 = inside/interior of that building

Japanese often leaves out words that are obvious from context. Instead of saying something longer like 建物の中 every time, the speaker can simply say 中 because it is already clear they mean the inside of the building.

Yes, 中は is enough because the context already tells us whose inside we are talking about.

The full idea would be:

• 建物の中は思ったよりきれいでした

But Japanese often omits information that is already understood. Since 建物 was just mentioned, saying only 中は sounds natural and efficient.

So both are understandable, but 中は is more natural here.

思ったより means than I thought or than expected.

Breakdown:

• 思った = thought (past form of 思う)
• より = than

Together, it works like a comparison:

• 思ったより きれいでした
• It was cleaner/prettier than I thought

This pattern is very common:

• 思ったより高い = more expensive than I thought
• 思ったより難しい = harder than I thought
• 思ったより小さい = smaller than I thought

Because 思ったより is a set pattern meaning than I thought.

Even in English, we often say than I thought, not than I think, when reacting to something we have now seen or experienced. Japanese does the same thing.

So:

• 思う = to think
• 思った = thought
• 思ったより = than I thought / than expected

This does not necessarily mean the whole event happened long ago. It just reflects the speaker’s earlier expectation.

Here, きれい most naturally means clean or neat, though it can also mean pretty or beautiful depending on context.

Because the sentence is about the inside of an old building, the most likely meaning is:

• cleaner / nicer-looking than expected

So きれいでした here suggests the interior was in better condition than the speaker expected.

No, きれい is not an i-adjective, even though it ends in い.

It is a na-adjective.

That means its polite past form is made with です:

• きれいです = is clean / is pretty
• きれいでした = was clean / was pretty

Compare:

• 古いです → 古かったです for an i-adjective
• きれいです → きれいでした for a na-adjective

So the い at the end of きれい is just part of the word, not the adjective ending pattern.

Because the speaker is describing two different parts of the sentence:

• けっこう古い建物です = It is a fairly old building
• 中は思ったよりきれいでした = The inside was cleaner than I thought

The first part presents the building in a general way, almost like identifying or describing it. The second part talks about the speaker’s impression of the inside, often after seeing it, so the past tense でした is natural.

In real Japanese, tense can sometimes follow the speaker’s perspective rather than strict English-style consistency. This sentence sounds natural.

Yes. Japanese often omits subjects when they are obvious from context.

In English, we might say:

• It’s a fairly old building, but the inside was cleaner than I thought.

In Japanese, the speaker does not need to say それは or repeat その建物は if everyone already knows what is being talked about.

So the understood subject is something like:

• this/that building
• or simply it

A simple breakdown is:

• けっこう = fairly / quite
• 古い = old
• 建物 = building
• です = is
• が = but
• 中は = as for the inside
• 思ったより = than I thought
• きれいでした = was clean / was pretty

So the structure is:

• It is a fairly old building, but as for the inside, it was cleaner than I thought.

This is a good example of how Japanese often puts descriptive information before the noun, uses は to mark a topic, and leaves obvious subjects unstated.

Yes. A casual version could be:

• けっこう古い建物だけど、中は思ったよりきれいだった。

Changes:

• ですが → だけど for casual but
• きれいでした → きれいだった for casual past

You could also hear:

• けっこう古い建物だけど、中は思ったよりきれい。

That sounds a bit more conversational, depending on context.