demo, sono hanasi ha zyoudan de ha naku, honki datta sou desu.

Here, でも means but or however.

It connects this sentence to a previous idea and shows contrast. For example, if the previous sentence suggested that something sounded silly or unbelievable, でも introduces the surprising fact that it was actually serious.

So it is not the でも meaning for example. It is the conjunction meaning but.

は marks その話 as the topic of the sentence.

So その話は means something like:

• as for that story
• regarding that story
• that story, ...

The sentence is then making a comment about that topic: it was not a joke; apparently it was serious.

A learner might expect が, but は is natural here because the speaker is talking about that story as the thing under discussion, not emphasizing it as newly identified information.

話 is a broad word. Depending on context, その話 can mean:

• that story
• that talk
• that matter
• what was said
• that conversation/topic

So the exact English translation depends on the situation.

In this sentence, その話 probably refers to something someone said or proposed, and the point is that it was not meant as a joke.

冗談ではなく means not a joke or rather than a joke.

It comes from:

• 冗談だ = it is a joke
• 冗談ではない = it is not a joke
• 冗談ではなく = not being a joke / not a joke, and...

The final なく is the continuative form of ない, used to connect to what comes next.

So the structure is roughly:

• 冗談ではなく、本気だった
• it was not a joke, but serious

This is a fairly formal or written-sounding way to connect ideas.

In ではなく, the では comes from the negative form of だ.

For na-nouns and nouns such as 冗談 and 本気, the negative often appears as:

• ～ではない
• ～じゃない in casual speech

So:

• 冗談ではない = it is not a joke
• casual: 冗談じゃない

The は here is part of that standard pattern. It is not a separate topic marker in the same way as その話は.

So 冗談ではなく is the connected form of 冗談ではない.

Yes. 冗談じゃなくて would be a more casual version.

Compare:

• 冗談ではなく = more formal / written / careful
• 冗談じゃなくて = more conversational

Both mean roughly not joking / not a joke, but...

So the sentence could be made more casual as:

• でも、その話は冗談じゃなくて、本気だったそうです。

The meaning is basically the same, but the tone changes.

本気 means seriousness, being serious, or for real.

Since 本気 is a noun / na-noun-like word, it often behaves with だ:

• 本気だ = serious / in earnest
• 本気だった = was serious / was for real

So 本気だった means that the person or statement was genuine, serious, and not meant as a joke.

In natural English, it often comes out as:

• they were serious
• it was serious
• they meant it

だった is the past form of だ.

That means the speaker is reporting that, at that time, it was serious.

Compare:

• 本気だ = is serious
• 本気だった = was serious

In this sentence, the speaker is talking about a past statement, past event, or something they later found out about. So the past form is natural.

No. This そうです is the hearsay use, meaning:

• I hear that...
• apparently...
• they say...

So 本気だったそうです means:

• apparently, it was serious
• I heard that it was serious

This is different from the appearance use of そうです, as in:

• おいしそうです = it looks delicious

A good clue is what comes before it:

• after a plain form like だった, そうです usually means I heard that...
• after a stem like おいしそう, it means looks...

Because of そうです, the speaker is showing that this information is reported, not directly asserted from personal certainty.

So the speaker is not necessarily saying:

• I know for sure it was serious

Instead, they are saying something like:

• I heard that it was serious
• Apparently, it was serious

This creates a little distance between the speaker and the information.

Japanese often leaves the subject unstated when it is clear from context.

So 本気だった could mean:

• the person who said it was serious
• the proposal was serious
• the statement itself was meant seriously

English usually forces you to choose something like he was serious, she was serious, or they meant it, but Japanese often does not.

Here, the omitted subject would be understood from the surrounding context.

The comma helps separate the two contrasting parts:

• 冗談ではなく
• 本気だったそうです

It is like a pause:

• not a joke, apparently it was serious

Japanese punctuation is often flexible, but here the comma makes the contrast easier to read.

Yes, very strongly.

The sentence sets up a clear contrast:

• 冗談ではなく = not a joke
• 本気だった = it was serious

This is a very natural pairing in Japanese. It is similar to saying:

• It wasn’t said jokingly; they really meant it.

So the sentence emphasizes that what might have seemed unserious actually was genuine.

You could say 冗談じゃなかった, but the nuance and structure would be a little different.

• 冗談じゃなかった = it was not a joke
• 冗談ではなく、本気だった = not a joke, but serious

The original version is more elegant and explicitly connects the two contrasting ideas in one structure.

If you said:

• でも、その話は冗談じゃなかったそうです。

that would simply mean:

• But apparently that wasn’t a joke.

It is fine, but it does not state the second half it was serious as directly.

It is polite and somewhat neutral-to-formal.

Reasons:

• ではなく sounds more formal than じゃなくて
• そうです is polite
• overall sentence style is careful and standard

A more casual version might be:

• でも、その話は冗談じゃなくて、本気だったらしい。
• でも、その話、冗談じゃなくて本気だったんだって。

So the original is a good standard sentence for polite conversation or writing.