kono mondai ha sorehodo muzukasiku nai desu.

は marks 問題 as the topic of the sentence.

So この問題は means something like:

• as for this problem
• this problem, ...

The sentence is not just identifying the subject doing an action; it is making a comment about this problem. The comment is:

• それほど難しくないです = is not that difficult

So the structure is:

• この問題は = as for this problem
• それほど難しくないです = it is not that difficult

English often does not clearly separate topic and subject, but Japanese does this a lot.

それほど means to that extent, that much, or more naturally in English here, that.

So:

• それほど難しくないです = it is not that difficult

A key point: それほど is very often used with a negative expression.

Examples:

• それほど高くない = not that expensive
• それほど遠くない = not that far
• それほど忙しくない = not that busy

It softens the statement a bit. Instead of saying it is not difficult, it says it is not that difficult.

Because 難しい is an い-adjective, and when you make an い-adjective negative, you change -い to -く and add ない.

So:

• 難しい = difficult
• 難しくない = not difficult

This is the normal negative form of an い-adjective.

More examples:

• 高い → 高くない = not expensive / not high
• 寒い → 寒くない = not cold
• 面白い → 面白くない = not interesting

So 難しく is not a separate word by itself here; it is the form needed before ない.

Because い-adjectives do not make negatives by simply adding ない to the dictionary form.

You cannot say:

• 難しいない ✘

You must change 難しい like this:

• remove the final い
• add くない

So:

• 難しい → 難しくない

Then, to make it polite, you can add です:

• 難しくないです

This is just how い-adjectives are conjugated in Japanese.

The です makes the sentence polite.

Compare:

• この問題はそれほど難しくない。 = plain style
• この問題はそれほど難しくないです。 = polite style

This is very common in modern Japanese. Even though 難しくない already contains the negative, adding です makes the whole sentence sound polite and appropriate in many everyday situations.

So です here is not creating the negative; it is mainly adding politeness.

Yes. That is another polite negative form.

You can say:

• この問題はそれほど難しくないです。
• この問題はそれほど難しくありません。

Both mean basically the same thing: This problem is not that difficult.

Nuance:

• 難しくないです is very common in everyday speech
• 難しくありません can sound a little more formal or textbook-like

Both are correct.

Yes, they are similar in this kind of sentence.

For example:

• それほど難しくないです
• あまり難しくないです
• そんなに難しくないです

All can mean it is not that difficult.

Rough differences:

• それほど = to that extent; slightly more neutral or measured
• あまり = often used with negatives to mean not very
• そんなに = that much; very common in conversation

In many situations, these are close in meaning. A learner will often hear all three.

Very often, yes.

In everyday Japanese, それほど commonly appears with negatives such as:

• それほど難しくない
• それほど高くない
• それほど必要ではない

Using それほど in a positive sentence is possible in some contexts, but learners will most often meet it with negatives.

So this sentence is following a very common pattern:

• それほど + adjective/adverb + negative
• meaning not that ...

Japanese word order is more flexible than English, but not everything can move freely.

The standard order here is:

• この問題はそれほど難しくないです。

This is the most natural basic version.

You might sometimes hear emphasis changes in conversation, but for learners, it is best to keep the sentence in this order:

1. topic: この問題は
2. degree word: それほど
3. adjective in negative polite form: 難しくないです

That pattern is reliable and natural.

Yes, if it is already clear which problem you mean.

• この問題はそれほど難しくないです。 = This problem is not that difficult.
• 問題はそれほど難しくないです。 = The problem is not that difficult. / As for the problem, it is not that difficult.

Using この makes the reference specific: this problem.

Without この, the listener has to understand from context which problem you are talking about.

The spaces are just for learning purposes.

Normal Japanese writing would usually be:

• この問題はそれほど難しくないです。

Japanese is normally written without spaces between words. Textbooks and beginner materials often add spaces to help learners see the parts more clearly.

So the spaced version is not wrong as a teaching tool, but the normal written sentence has no spaces.