keikoku ha daizi desu.

It is read:

けいこく は だいじ です。
keikoku wa daiji desu

A few notes:

• 警告 = けいこく
• は is written ha but here it is pronounced wa
• 大事 = だいじ
• です is often pronounced lightly, almost like des in careful speech

は marks the topic of the sentence.

So 警告は means something like:

• as for warnings...
• speaking of warnings...

Then the rest of the sentence says something about that topic:

• 大事です = are important / is important

So the overall structure is:

• 警告は = As for warnings
• 大事です = they are important

This is different from English, where we usually just put the subject first without a special topic marker.

This is a special feature of Japanese particles.

When は is used as the topic particle, it is pronounced わ even though it is written は.

So:

• は as a normal syllable = ha
• は as a particle = wa

This is something learners just have to memorize.

Both は and が can sometimes look similar in English translations, but they do different jobs.

In this sentence, は is used because the speaker is presenting 警告 as the topic:

• 警告は大事です。 = As for warnings, they are important.

If you used が, the nuance would shift more toward identifying or emphasizing the subject:

• 警告が大事です。 = It is warnings that are important / Warnings are the important thing

So:

• は = topic, general statement, contrast-friendly
• が = subject marker, often more specific or focused

For a simple general statement like this, は is very natural.

大事 is a na-adjective.

Many learners expect adjectives to behave like い-adjectives, but 大事 is different. It belongs to the na-adjective group.

That means:

• before a noun: 大事な警告 = an important warning
• at the end of a sentence: 警告は大事です = Warnings are important

So in this sentence, 大事 is being used at the end of the sentence, followed by です, which is why there is no な.

Because な is only used when a na-adjective comes directly before a noun.

Compare:

• 大事な警告 = an important warning
  
  • here 大事 modifies the noun 警告, so you need な

But:

• 警告は大事です = Warnings are important
  
  • here 大事 is the predicate, so you do not use な

This is a very common pattern with na-adjectives.

です makes the sentence polite, but it is not exactly the same as English is.

In beginner translations, it often gets translated as:

• is
• are

So 大事です is often translated as is important or are important.

But in Japanese grammar, です is better thought of as a politeness marker used at the end of a sentence.

Compare:

• 警告は大事。 = plain / casual
• 警告は大事です。 = polite

So です helps make the sentence sound more formal or polite.

It can be either, depending on context.

Japanese nouns usually do not show singular/plural in the same way English nouns do.

So 警告 could mean:

• a warning
• the warning
• warnings

Context tells you which one is meant.

That is why a sentence like 警告は大事です could be understood in English as:

• A warning is important
• Warnings are important
• The warning is important

The exact translation depends on the situation.

The spaces are probably there just to help learners see the parts clearly.

Normal Japanese is usually written without spaces:

警告は大事です。

Japanese readers naturally recognize where words and particles begin and end by:

• kanji
• hiragana
• grammar patterns
• context

So the spaced version is a teaching aid, not the usual way Japanese is written.

Yes, it is a complete sentence.

Japanese does not always need a verb in the same way English does. A sentence can end with:

• a noun
• an adjective
• a na-adjective + です
• a verb

Here, the sentence ends with 大事です, which works as the predicate.

So even though English wants something like is important, Japanese can express that idea with:

• 大事です

That is a full, normal sentence.

Yes. The normal written form is:

警告は大事です。

A few small points:

• no spaces in normal Japanese
• Japanese usually uses 。 instead of the English period .
• the sentence is polite and straightforward

So the learner-spaced version and the normal version are the same sentence, just presented differently:

• learner-friendly: 警告 は 大事 です。
• normal Japanese: 警告は大事です。