masui ha hituyou desu.

You’re right: normal Japanese writing usually does not use spaces between words.

So this would normally be written as:

麻酔は必要です。

Spaces are often added in teaching materials to make the structure easier to see for beginners. They help you notice the three parts:

• 麻酔 = anesthesia
• は = topic marker
• 必要です = is necessary

It is read:

ますい は ひつよう です。

Romanized, that is:

Masui wa hitsuyou desu.

A couple of pronunciation notes:

• は is written ha, but when it is the topic particle, it is pronounced wa.
• です is often pronounced lightly, something like des in everyday speech, though learners can safely say desu.

は marks the topic of the sentence.

So 麻酔は必要です is roughly:

• As for anesthesia, it is necessary.

In more natural English, that becomes:

• Anesthesia is necessary.

The topic marker は tells you what the sentence is about. It does not exactly equal English is.

Both can appear with 必要, but the nuance is a little different.

• 麻酔は必要です。
  As for anesthesia, it is necessary.
  This presents 麻酔 as the topic.

• 麻酔が必要です。
  Anesthesia is what is necessary / Anesthesia is needed.
  This puts more focus on 麻酔 itself as the thing that is needed.

For a beginner, a simple way to think about it is:

• は = talking about anesthesia as the topic
• が = emphasizing that anesthesia is the needed thing

Both are natural depending on context.

必要 is a na-adjective (also called an adjectival noun).

It means necessary.

That is why it can work like this:

• 必要です = is necessary

And before a noun, it takes な:

• 必要な手術 = a necessary surgery
• 必要なもの = necessary thing(s)

So in your sentence, 必要 is acting as the main descriptive word: necessary.

Japanese often expresses ideas differently from English.

English commonly says:

• We need anesthesia.

Japanese can instead say:

• 麻酔は必要です。
• literally, Anesthesia is necessary.

So rather than using a verb like to need, Japanese often uses 必要 (necessary) to describe the situation.

A related pattern you will see a lot is:

• Noun + が/は + 必要です
• X is necessary
• X is needed

です makes the sentence polite and completes it in a standard polite style.

So:

• 麻酔は必要です。 = polite
• 麻酔は必要。 = casual / plain, often used in conversation or notes

For learners, it is safest to use です in most everyday situations unless you know a casual style is appropriate.

Yes, Japanese often leaves out information that is understood from context.

In 麻酔は必要です, the sentence does not explicitly say:

• who needs it
• for what procedure
• for whom it is necessary

Depending on context, it could mean things like:

• Anesthesia is necessary.
• You will need anesthesia.
• Anesthesia is required.

Japanese frequently omits subjects and other details when they are already clear.

Because 必要です is the predicate of the sentence.

The structure is:

• 麻酔 = topic
• は = topic marker
• 必要です = is necessary

You do not need another particle between 必要 and です here.

Think of 必要です as one unit meaning:

• is necessary

Yes.

Casual

• 麻酔は必要。
• 麻酔が必要。

Standard polite

• 麻酔は必要です。
• 麻酔が必要です。

More formal/stiff

• 麻酔は必要です。 is already polite, but in formal writing you might also see:
• 麻酔が必要となります。
• 麻酔が必要です。

The exact choice depends on situation, but 必要です is a very common and useful polite form.

麻酔 is read ますい and means anesthesia.

The kanji are:

• 麻
• 酔

For a beginner, the most important thing is simply to recognize that:

• 麻酔 = ますい = anesthesia

You do not need to fully analyze the kanji to understand the sentence, but recognizing the whole word as one medical term is very helpful.

Yes. Depending on context, very natural English translations include:

• Anesthesia is necessary.
• Anesthesia is needed.
• Anesthesia is required.

The Japanese sentence itself is fairly neutral. The best English wording depends on the situation:

• medical explanation
• consent form
• doctor speaking to a patient
• discussion of a procedure

So even if one translation is shown to the learner, several English versions may fit.

Yes, it is a complete sentence.

Japanese does not need as many explicit elements as English does. Even though it may feel short, 麻酔は必要です。 is fully grammatical and natural.

It has everything needed:

• a topic: 麻酔
• a statement about that topic: 必要です

That kind of compact sentence is very common in Japanese.