matto ha kirei desu.

は marks the topic of the sentence.

So in マット は きれい です, マット is what the sentence is about, and the rest of the sentence says something about it.

A natural way to think about it is:

• As for the mat, it is clean.
• or simply The mat is clean.

Even though は is often translated as is in very simple beginner sentences, it does not literally mean is. Its real job is to show the topic.

This is one of the first unusual things Japanese learners notice.

When は is used as a particle, it is pronounced wa, not ha.

So:

• マット は きれい です
• is read matto wa kirei desu

But when は is part of a normal word, it is usually pronounced ha.

This is just a special rule for the particle は, and you will see it very often.

In this sentence, there is no main action verb like eat, run, or see.

Instead, きれい is a na-adjective meaning something like clean or pretty, and です makes the sentence polite and complete.

So Japanese can make a full sentence like this without a separate verb such as to be in the same way English does.

You can think of:

• マット は きれい です
• as The mat is clean

But Japanese is not literally building the sentence in exactly the same way English does.

きれい can mean both clean and pretty / beautiful, depending on context.

Common uses include:

• 部屋はきれいです。 = The room is clean.
• 花はきれいです。 = The flowers are pretty.

So with マット, the intended meaning depends on the context you were given.

This is very common in Japanese: one word may have a range of related meanings, and context tells you which one is most natural.

です makes the sentence polite.

Compare:

• マット は きれい です → polite
• マット は きれい → plain / casual

In beginner translations, です is often explained as is, but that is only partly helpful. It is better to think of it as a polite sentence ending that helps make statements sound proper and natural in polite speech.

Yes, きれい is an adjective, but it is a na-adjective, not an i-adjective.

This is important because even though きれい ends in the sound -ei, it is not an i-adjective.

For example:

• きれいな マット = a clean/pretty mat
• きれいです = is clean / is pretty

By contrast, a true i-adjective would behave differently, such as:

• あついです = is hot
• あつい マット = a hot mat

So きれい is one of the adjectives you simply have to learn as a na-adjective.

マット is written in katakana because it is most likely a loanword from English, mat.

Katakana is commonly used for:

• foreign loanwords
• foreign names
• some sound effects
• emphasis in certain cases

So if the word means mat, katakana makes perfect sense.

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

So this sentence would normally be written as:

• マットはきれいです。

The spaces in your example are probably there to help beginners see the parts more clearly:

• マット
• は
• きれい
• です

This is a teaching aid, not standard everyday writing.

Sometimes yes, but the nuance changes.

• マットはきれいです = As for the mat, it is clean
• マットがきれいです = The mat is the one that is clean or It is the mat that is clean

Very roughly:

• は introduces the topic
• が puts more focus on the subject itself

For a first sentence out of nowhere, は is often the most natural if you are simply talking about the mat.
が can sound more specific or contrastive depending on context.

Yes, in casual speech you often can.

• マットはきれいです。 → polite
• マットはきれい。 → casual

Both can be correct, but the polite version is safer for beginners and more appropriate in formal situations.

The basic word order here is already normal and natural.

Japanese often follows this pattern:

• topic + comment

So:

• マットは = topic
• きれいです = comment about the topic

That is one reason this sentence is a very common beginner pattern. It is simple, natural, and useful for many descriptions:

• へやはきれいです。 = The room is clean.
• そのひとはしんせつです。 = That person is kind.
• このまちはしずかです。 = This town is quiet.