nendo no seikei ha muzukasii.

It is read:

ねんど の せいけい は むずかしい。

• 粘土 = ねんど
• 成形 = せいけい
• 難しい = むずかしい

So the whole sentence is:

ねんど の せいけい は むずかしい。

粘土の成形 is a noun phrase.

• 粘土 = clay
• 成形 = shaping, molding, forming
• 粘土の成形 = the shaping/forming of clay, clay molding

The particle の connects the two nouns, so 粘土 modifies 成形.

Because both 粘土 and 成形 are nouns.

In Japanese, noun + の + noun is a very common way to show a relationship like:

• possession
• category
• material
• target
• subject matter

Here, 粘土の成形 means something like:

• shaping of clay
• clay shaping
• clay molding

So の is linking clay to shaping/forming.

は marks 粘土の成形 as the topic:

• As for clay molding, it is difficult.

This gives the sentence a general, topic-comment structure.

If you used が, the sentence would sound more like you are identifying that specific thing as what is difficult:

• 粘土の成形が難しい。
• It is clay molding that is difficult. / Clay molding is difficult.

Both are possible, but は feels more like a general statement or description of the topic.

Because this is in the plain form:

• 粘土の成形は難しい。

Japanese i-adjectives like 難しい can end a sentence by themselves.

If you want to make it more polite, you add です:

• 粘土の成形は難しいです。

Both mean the same thing, but the level of politeness is different.

Grammatically, 難しい is an i-adjective.

In Japanese, i-adjectives can function as the predicate of a sentence, so they can behave a bit like English verbs such as is difficult.

That is why:

• 粘土の成形は難しい。

is a complete sentence, even without a separate word for is.

It most directly means:

• Clay molding is difficult
• The shaping of clay is difficult

In natural English, depending on context, it could also be expressed as:

• It is difficult to shape clay
• Shaping clay is difficult

So the Japanese focuses on the noun phrase 粘土の成形 rather than using a verb like to shape.

成形 means forming, shaping, or molding something into a shape.

It is often used in:

• manufacturing
• crafts
• technical explanations
• materials processing

So it can sound a bit more technical or formal than a simpler everyday phrasing.

For example, in casual speech someone might say something more like:

• 粘土を形にするのは難しい。
• 粘土で形を作るのは難しい。

But 粘土の成形は難しい is perfectly natural, especially in an instructional, technical, or written context.

Yes, absolutely.

• 粘土の成形は難しい。
• 粘土を成形するのは難しい。

Both are natural, but the structure is different.

粘土の成形は難しい

This is more noun-based and a bit more formal or technical.

粘土を成形するのは難しい

This is more verb-based and may feel more straightforward to learners:

• To mold clay is difficult
• Molding clay is difficult

So the original sentence packages the action into the noun 成形.

Because the sentence does not use a verb phrase like to mold clay.

Instead, it uses the noun phrase:

• 粘土の成形 = the molding of clay

Since 成形 is being used as a noun here, you do not need を.

If you changed it into a verb expression, then を would appear:

• 粘土を成形するのは難しい。

There, 粘土 is the object of 成形する.

In this sentence, 難しい clearly means difficult.

Depending on context, 難しい can also mean things like:

• hard to understand
• complicated
• delicate
• problematic

For example, it can sometimes mean sensitive or not easy to handle in a broader sense.

But here, with 粘土の成形, the natural reading is simply:

• difficult
• hard to do

It is a plain non-past sentence.

Japanese non-past often covers:

• present
• general truths
• habitual situations
• future, depending on context

Here it sounds like a general statement:

• Clay molding is difficult.

It is not specifically about one single moment unless the context makes it so.

Yes, it sounds natural, especially in written, instructional, or somewhat technical Japanese.

It could appear in contexts like:

• an art textbook
• a craft manual
• a materials science explanation
• a comment about pottery or sculpture

In everyday conversation, people might choose a slightly less technical wording, but the original sentence is still correct and natural.