kono syugi ha taisetu desu.

この means this and it must be followed by a noun.

So:

• この主義 = this principle / this doctrine / this ideology

In Japanese, この is a demonstrative determiner, so it works like English this before a noun. It does not stand alone.

Compare:

• この主義 = this principle
• これ = this one / this thing

So you use この because it is directly modifying 主義.

主義 is read しゅぎ.

It often means something like:

• principle
• doctrine
• ideology
• -ism in some contexts

Its exact meaning depends on context. For example:

• 民主主義 = democracy
• 資本主義 = capitalism
• 平和主義 = pacifism
• 主義 by itself can mean a guiding principle, doctrine, or belief system

So in this sentence, 主義 sounds fairly abstract and formal. It is not a casual everyday noun.

は marks the topic of the sentence.

So:

• この主義は = as for this principle...

The sentence is not just saying what the subject does; it is presenting this principle as the thing being talked about, and then making a comment about it:

• この主義は大切です。
• As for this principle, it is important.

This topic-comment structure is very common in Japanese.

This is a standard feature of Japanese particles.

The character は is normally read ha, but when it is used as the topic particle, it is pronounced wa.

So:

• この主義は大切です is pronounced roughly:
• kono shugi wa taisetsu desu

This is something learners simply need to memorize.

Yes, 大切 is a na-adjective.

But na-adjectives only use な when they come directly before a noun.

Examples:

• 大切な本 = an important book
• 大切な考え方 = an important way of thinking

When a na-adjective is used as the predicate at the end of a sentence, you do not use な:

• この主義は大切です。 = This principle is important.

So:

• before a noun: 大切な
• at the end of a sentence: 大切です / 大切だ

です makes the sentence polite and acts like a polite copula here.

So:

• この主義は大切です。 = polite
• この主義は大切だ。 = plain style

In this sentence, です does not add a separate meaning like do or is in a one-to-one way; it mainly gives the sentence polite structure.

Yes, it is different from normal English word order.

Japanese often works like this:

• topic + comment

So the structure is:

• この主義は = as for this principle
• 大切です = is important

A very literal gloss would be:

• This principle, as for it, important is

Natural English becomes:

• This principle is important.

So the Japanese sentence is completely normal, even though the order feels different to English speakers.

Japanese does not have articles like a, an, and the.

That means この主義 already gives enough information:

• この tells you it is this specific principle

Because of that, no extra article is needed.

Depending on context, English may translate the sentence as:

• This principle is important.
• This doctrine is important.
• This ideology is important.

Japanese leaves some of these article-related choices to context.

Grammatically, yes — it is correct and natural.

However, 主義 is a fairly formal and abstract word, so the sentence sounds like something from:

• a discussion of ideas
• politics
• philosophy
• education
• ethics

If the context is about beliefs, doctrines, or principles, it sounds fine.

Without context, it can feel a little formal or textbook-like simply because 主義 is not a very casual everyday word.

In normal Japanese writing, you usually would not put spaces between these words.

It would normally be written as:

• この主義は大切です。

Spaces are often added in beginner materials to make the structure easier to see.

So the spaced version is just a learning aid, not standard Japanese formatting.

Yes. The plain-style version is:

• この主義は大切だ。

You may also sometimes hear:

• この主義は大切。

That last version is more casual and conversational.

So the differences are mainly about style and politeness:

• 大切です = polite
• 大切だ = plain
• 大切 = casual / conversational omission

Usually, は is the more natural choice here because the sentence is making a general statement about this principle as a topic.

• この主義は大切です。 = As for this principle, it is important.

If you used が, it would sound more like you are specifically identifying this principle as the important one, often in contrast to others:

• この主義が大切です。 = This principle is the important one.

So:

• は = topic, neutral statement about it
• が = stronger focus or identification

For a simple sentence like this, は is the default choice.