Learn Hardcore Japanese: sankatouroku ha muryou nanoni, naiyou ha totemo kuwasii. - Registration is free, yet the content is very detailed.

It means “even though/despite,” with a nuance of surprise or contradiction. The clause before なのに sets an expectation; the clause after it states something that goes against that expectation. Here: “Even though registration is free, the content is very detailed (surprisingly).”

Why is it なのに after 無料 and not just のに or だのに?

Because のに attaches to a plain-form clause. With nouns and na‑adjectives, you use な before のに: noun/na‑adj + なのに.

Noun/na‑adj: 無料なのに
i‑adj: 高いのに
Verb: あるのに Using だのに after a noun/na‑adj is not standard; stick with なのに.

What part of speech is 無料, and how do I use it?

無料 is a noun that behaves like a na‑adjective.

Predicate: 登録は無料だ／です (Registration is free.)
Attributive: 無料の登録 (free registration)
Adverbial/means: 無料で登録する (register for free) In the target sentence it’s followed by なのに, so you need the な: 無料なのに.

Why are there two は’s?

They mark two contrastive topics:

参加登録は (as for registration) … 無料
内容は (as for the content) … とても詳しい This “Aは…, Bは…” structure highlights a contrast: free registration vs. very detailed content.

Could I use が instead of は here?

You can, with nuance shifts:

参加登録が無料なのに、内容はとても詳しい。 Focuses on the fact that it’s (specifically) the registration that’s free (more “new information” feel).
…内容がとても詳しい。 Makes “the content is detailed” feel like a factual description rather than a contrastive topic.
All versions are grammatical; は emphasizes contrast, が emphasizes identification.

How is なのに different from けど, が, and でも?

なのに: strong “despite/contrary to expectations.”
けど: milder “but/though,” often conversational.
が: “but” within a sentence; more formal/neutral than けど.
でも: “but/however” linking separate sentences or starting a sentence.
Example gradation:
無料なのに、内容は詳しい。 (surprising)
無料だけど、内容は詳しい。 (plain contrast)
無料ですが、内容は詳しいです。 (polite contrast)

Can I say 無料でも instead of 無料なのに?

無料でも means “even if it’s free” (conditional/concessive), not “even though (it actually is) free.” If the freeness is a known fact and you want the “despite” nuance, 無料なのに is the natural choice.

無料でも、内容は詳しい。 = Even if it’s free (hypothetically or as a general rule), the content is detailed.
無料なのに、内容は詳しい。 = It’s free, yet (surprisingly) the content is detailed.

Is 詳しい the right word for “detailed” when talking about content?

Yes. 詳しい (i‑adj) works for explanations, articles, and content: 内容は詳しい／説明が詳しい.
Alternatives:

詳細な (na‑adj) / 詳細だ (noun/na‑adj): more formal/literary.
充実している: “substantial/fulfilling,” focusing on richness rather than fine detail.
濃い: colloquial “dense,” context‑dependent.

What’s the difference between 詳しい and 詳しく?

詳しい is an adjective: 説明は詳しい (The explanation is detailed).
詳しく is the adverb form: 詳しく説明する (to explain in detail).

How do I make the sentence polite?

Add です to the predicate(s):

参加登録は無料なのに、内容はとても詳しいです。
If you want a gentler contrast, use ですが:
参加登録は無料ですが、内容はとても詳しいです。

Is 参加登録 redundant? Could I just say 参加, 登録, or 申し込み?

Each has a nuance:

参加登録: “registration to participate” (common for events).
参加: “participation/attending”; not specifically “sign‑up.”
登録: “registration” in general (accounts, databases, systems).
申し込み／申込: “application/sign‑up” (the act of applying).
参加申し込み is also common. Choose based on whether you mean “signing up to take part” (参加登録/参加申し込み) vs. “registration” more broadly (登録).

Are spaces and the comma used here normal in Japanese?

In standard Japanese, you don’t insert spaces between words. The comma 、 and period 。 are normal. Written naturally: 参加登録は無料なのに、内容はとても詳しい。

Can I reverse the clause order?

Yes: 内容はとても詳しいのに、参加登録は無料。

Original: highlights how detailed the content is despite being free.
Reversed: highlights that it’s free despite the content being so detailed.
Both are grammatical; the main point tends to be the clause after the connector.

Does のに carry a complaining tone?

It can, depending on context and what follows. If the second clause is negative or expresses dissatisfaction, it can sound like a complaint: 手伝ったのに、感謝されなかった。 Here, the second clause is positive, so it reads as pleasant surprise rather than complaint.

How do I read the key words?

参加登録: さんかとうろく (sanka tōroku)
無料: むりょう (muryō)
内容: ないよう (naiyō)
詳しい: くわしい (kuwashii)

Is のに literally の + に here?

No. のに is a fixed concessive conjunction meaning “even though/though.” The の here is the nominalizer that allows nouns/na‑adjectives to connect (hence なのに), not the possessive の.