Learn Hardcore Japanese: kono kaigi de ha masukutyakuyou ga hituyou desu. - In this meeting, wearing a mask is necessary.

If you used に instead (この会議に), it would sound more like “to this meeting” or “for this meeting” (direction or target), which doesn’t fit as naturally when stating a rule that applies within the meeting.

So for “At this meeting, X is required/it happens,” で is the normal choice.

Why do we have both で and は together as では? What is では doing here?

Here you actually have two particles in a row:

で = location of the event (at this meeting)
は = topic marker (as for / regarding)

So この会議では can be understood as:

“As for this meeting, at it, mask-wearing is necessary.”

The は makes この会議で the topic of the sentence. It can also add a slight contrast, like “At this meeting (as opposed to others), mask-wearing is required.”

You’ll often see this written with no space: この会議ではマスク着用が必要です。

Why is マスク written in katakana?

マスク is a loanword from English “mask”, so it’s written in katakana, which is the standard script for:

Foreign loanwords (テレビ, コーヒー, コンピュータ)
Some onomatopoeia, brand names, emphasis, etc.

There is a native word 仮面 (kamen), but that usually means a theatrical mask, disguise mask, etc., not a medical or surgical mask. For everyday “mask” (especially medical masks), マスク in katakana is the normal word.

What does 着用 (ちゃくよう) mean, and why not just say マスクが必要です?

着用 means “wearing” (putting on and having something on, like clothes, a helmet, a seatbelt).

マスク着用が必要です。
= “Mask wearing is necessary.”
マスクが必要です。
= “Masks are necessary.” (the masks themselves are needed)

In practice, both could be understood as “You must wear a mask,” but:

マスク着用が必要です is more formal and emphasizes the act of wearing.
マスクが必要です focuses on having masks, and could sound slightly more like “You need to have a mask” (for example, to enter).

In notices and rules, ～着用 is very common:

安全ベルト着用 = wearing a seatbelt
ヘルメット着用 = wearing a helmet

Why is there が before 必要です? What does マスク着用が必要です literally mean?

The structure is:

[Noun phrase] + が + 必要です

Here:

マスク着用 = mask wearing
が = subject marker
必要です = is necessary / is needed

So literally:

マスク着用が必要です。
= “Mask wearing is necessary.”
= “It is necessary to wear a mask.”

This pattern is very common:

予約が必要です。 = A reservation is necessary. / You need a reservation.
パスポートが必要です。 = A passport is necessary. / You need a passport.

Is 必要 (ひつよう) a verb or an adjective? How does 必要です work grammatically?

必要 is a noun / na-adjective (形容動詞), not a verb.

As a na-adjective / noun, it combines like this:

必要だ / 必要です = is necessary
必要な書類 = necessary documents

Patterns you’ll see:

N が必要です。 = N is necessary / You need N.
V-るひつようがあります。 (more formal)
V-る必要があります。 = There is a need to do V.

In your sentence, 必要です just means “is necessary” in polite form.

Could I say マスクを着用する必要があります instead? What’s the difference?

Yes, that is also correct and very natural:

マスクを着用する必要があります。
= “There is a need to wear a mask.”
= “You need to wear a mask.”

Difference in feel:

マスク着用が必要です。
- Noun phrase + が + 必要です
- Very concise, notice-like, formal.
- Reads like a rule label: “Mask-wearing is required.”
マスクを着用する必要があります。
- Full clause (マスクを着用する) + 必要があります
- Slightly more explicit and sentence-like.
- Common in spoken polite language and more detailed written instructions.

Both communicate essentially the same requirement.

Why is there no explicit “you” or “we” in the sentence?

Japanese often omits pronouns when they’re obvious from context.

In English, we might say:

“You must wear a mask at this meeting.”
“Everyone must wear a mask at this meeting.”

In Japanese, the rule can be stated more impersonally, focusing on the condition itself:

この会議ではマスク着用が必要です。
= “At this meeting, mask-wearing is required.”

The listener naturally understands that participants / attendees are the ones who have to follow the rule, even though “you” or “everyone” is not directly said.

Can I drop です and just say マスク着用が必要?

Grammatically, you’d normally finish with だ / です in a full sentence:

マスク着用が必要だ。 (plain)
マスク着用が必要です。 (polite)

マスク着用が必要 without だ/です feels incomplete in ordinary speech or writing, unless:

It’s part of a longer phrase (e.g., マスク着用が必要な会議 = a meeting where mask-wearing is necessary), or
It’s in a very telegraphic style (a headline, a note on a form, etc.).

So as a normal sentence to someone, keep です (polite) or だ (plain).

Could I say この会議はマスク着用が必要です without で? What changes?

Yes, you can say:

この会議はマスク着用が必要です。

Then the structure is:

この会議は = As for this meeting,
マスク着用が必要です。 = mask-wearing is necessary.

Meaning-wise, it’s almost the same. Subtle difference:

この会議では
- Emphasizes the setting/inside the meeting (“at this meeting / during this meeting”).
この会議は
- Treats this meeting more as the topic as a whole; sounds like “This meeting (as an event) requires mask-wearing.”

In many everyday contexts, both are acceptable and will be understood the same.