yuugaigomi ha abunai desu.

有害ごみ is read ゆうがいごみ.
は is the topic particle, so here it is pronounced わ, not は.
危ない is read あぶない.
です is read です.

So the full reading is:

ゆうがいごみ は あぶない です。

When は is used as a particle, it is pronounced わ.

So in this sentence:

• 有害ごみは = ゆうがいごみは, but pronounced ゆうがいごみわ

This is one of the most important pronunciation rules beginners learn in Japanese.

は marks the topic of the sentence.

The structure is basically:

X は Y です
= As for X, it is Y

So here, 有害ごみ is the topic, and the sentence is making a statement about it.

Using は gives the feeling of a general statement or description about hazardous trash.

危ない is an い-adjective. It already carries the meaning of being dangerous, so technically it can end a sentence by itself:

有害ごみは危ない。

Adding です makes the sentence more polite:

有害ごみは危ないです。

A useful point:

• with nouns: 学生です
• with な-adjectives: 静かです
• with い-adjectives: 危ないです

In the last case, です adds politeness, but 危ない is still the main descriptive word.

Yes, it is completely normal.

A beginner may wonder whether です can come after an い-adjective, and the answer is yes. This is standard polite Japanese.

Examples:

• 暑いです
• 高いです
• 危ないです

So 危ないです is a very common pattern.

You could, but the nuance changes.

• 有害ごみは危ないです。
  This sounds like a general statement about hazardous trash.

• 有害ごみが危ないです。
  This puts more focus on hazardous trash itself, as if answering What is dangerous?

For a simple general description, は is the more natural choice.

Yes, it functions as one noun phrase, and you can think of it as a compound noun:

• 有害 = harmful, hazardous
• ごみ = garbage, trash

Together, 有害ごみ means the category of hazardous waste or hazardous trash.

Because it acts like a noun, you can attach particles directly to it:

• 有害ごみは
• 有害ごみを
• 有害ごみに

That is very common.

In modern Japanese, ごみ is often written in:

• hiragana: ごみ
• katakana: ゴミ

The kanji for this word exists, but it is not commonly used in everyday writing.

So 有害ごみ looks very natural, especially in practical writing such as signs, notices, and garbage-sorting labels.

Both can work, but they feel a little different.

• 危ない is a very common everyday adjective
• 危険 is more formal and often appears in warnings or official notices

So:

• 有害ごみは危ないです。 sounds natural and straightforward
• 有害ごみは危険です。 sounds a bit more formal or official

A city notice or warning label might prefer 危険 more often, but 危ない is still perfectly understandable.

They are almost certainly there for learners.

Normally, Japanese is written without spaces:

有害ごみは危ないです。

Textbooks, children's materials, and beginner learning content sometimes add spaces to make the sentence easier to read.

Usually it means hazardous trash in general.

Japanese often does not mark singular vs. plural the way English does, and it also does not use articles like a or the.

So 有害ごみ can refer to the whole category, depending on context. In this sentence, it sounds like a general statement about that type of trash.