Learn Hardcore Japanese: kanozyo ha kaigi no naiyou ni kuwasii desu. - She is knowledgeable about the meeting's content.

With the adjective 詳しい (“knowledgeable; well-versed; detailed”), the thing you’re knowledgeable about is marked by に. So the pattern is: N に詳しい.

Examples: ITに詳しい, 日本史に詳しい, ワインに詳しい.
Don’t use を or で here: 内容を詳しい/内容で詳しい are ungrammatical.
You can also say N について詳しい, but see the nuance below.

Is 詳しい an i-adjective or a na-adjective? How do I conjugate it?

詳しい is an i-adjective.

Plain present: 詳しい
Polite present: 詳しいです
Negative: 詳しくない / 詳しくありません
Past: 詳しかった（です）
Negative past: 詳しくなかった（です）/ 詳しくありませんでした
Adverb/te-form: 詳しく, 詳しくて Note: You cannot say 詳しいだ (that’s wrong). Use 詳しい (plain) or 詳しいです (polite).

Why is there の between 会議 and 内容?

の links nouns and often corresponds to “of”: 会議の内容 = “the content of the meeting.”
You will also see the compound 会議内容 in headings or documents; it’s concise and a bit label-like. In everyday speech, 会議の内容 sounds more natural.

What’s the difference between N に詳しい and N について詳しい?

N に詳しい: The standard, compact way to say someone is well-versed in a field/topic. Feels natural for expertise.
Ex: 法律に詳しい (well-versed in law).
N について詳しい: “Detailed regarding N.” Very common when modifying nouns (e.g., Nについての詳しい説明). It can also describe a person, but is a bit wordier.
In this sentence, 会議の内容に詳しい pinpoints the content as the domain of expertise. 会議の内容について詳しい is possible but heavier.
Be careful: 会議について詳しい shifts the meaning to “knowledgeable about the meeting (as an event)” rather than specifically its content.

Could I say 詳しく知っている or よく知っている instead?

Yes, both are natural, with slight nuance:

詳しい states a quality (“is well-versed”).
よく知っている emphasizes knowing well.
詳しく知っている emphasizes knowing in detail. Examples:
彼女は会議の内容に詳しいです。
彼女は会議の内容をよく知っています。
彼女は会議の内容を詳しく知っています。 Note the particle change: with 知っている, use を, not に.

Why use は after 彼女? Could I use が instead?

彼女は… sets “she” as the topic/background: “As for her, she’s familiar with…”
彼女が… marks “she” as the specific subject and is used when identifying or contrasting: “It’s she who is familiar…” For example, in answer to “Who knows the meeting’s content?” you’d naturally use 彼女が会議の内容に詳しいです.

Can I drop です or replace it with だ?

Dropping です makes it plain/casual: 彼女は会議の内容に詳しい。
You cannot use だ after an i-adjective: 詳しいだ is incorrect.
To stay polite, use 詳しいです.

Can I reorder the sentence?

Yes, word order is flexible for emphasis:

会議の内容に彼女は詳しいです。 (fronts what she’s knowledgeable about)
会議の内容は彼女が詳しいです。 (as for the content, she’s the one who knows it) Avoid double-は like 会議の内容は彼女は…; it’s awkward.

Does 彼女 mean “she” or “girlfriend” here? Should I avoid pronouns?

彼女 can mean either “she/her” or “girlfriend,” depending on context. In neutral explanatory sentences like this, it reads as “she.” In real conversation, Japanese often omits pronouns or uses names/titles: 田中さんは会議の内容に詳しいです。

How do I say the negative (or past) version?

Negative (polite): 彼女は会議の内容に詳しくないです / 詳しくありません。
Past (polite): 彼女は会議の内容に詳しかったです。
Negative past (polite): 彼女は会議の内容に詳しくなかったです / 詳しくありませんでした。

What’s the opposite of 詳しい?

Neutral negation: 詳しくない (not familiar/detailed).
A lexical opposite: 疎い（うとい） = “unfamiliar; not well-versed.” Pattern: N に疎い
Ex: 彼女は会議の内容に疎いです。 (She isn’t versed in the meeting’s content.)

What other nouns work with …に詳しい?

Many domains/topics:

ITに詳しい, 経済に詳しい, 日本史に詳しい, ワインに詳しい, 法律に詳しい, この町の歴史に詳しい.

How do I ask “Who is familiar with the content of the meeting?”

Two natural options:

誰が会議の内容に詳しいですか。
会議の内容に詳しいのは誰ですか。
The second emphasizes the topic (“as for being familiar with the content, who is it?”).

If I want to say “the explanation is detailed,” can I say 内容が詳しい?

Better alternatives:

For explanations/descriptions: 詳しい説明, 説明が詳しい.
For content itself, Japanese often uses: 内容が詳細だ／具体的だ／充実している rather than 内容が詳しい.
Use 詳しい directly before nouns (e.g., 詳しい説明, 詳しい地図) or with に詳しい for a person’s expertise.

Does 内容 mean the minutes of the meeting?

No. 内容 = “content” (what was discussed/decided).

“Minutes (the written record)” = 議事録
“Agenda (items to be discussed)” = 議題
So your sentence says she knows the content, not that she has the minutes.