atarasii syouhin no kaihatu ga hazimatta.

A natural breakdown is:

• 新しい = new
• 商品 = product / merchandise
• の = links nouns, often like of
• 開発 = development
• が = marks the subject of the verb
• 始まった = started / began

So the structure is:

新しい 商品 の 開発 が 始まった。
= The development of a new product started.

In Japanese, adjectives normally come before the noun they describe.

So:

• 新しい商品 = a new product

This is similar to English word order:

• new product

The adjective 新しい directly modifies 商品.

Here, の connects two nouns.

• 商品 = product
• 開発 = development
• 商品の開発 = development of a product

A very common way to think about の is of, although it does more than that in Japanese.

In this sentence, 新しい商品の開発 means:

• the development of a new product

So の is showing the relationship between product and development.

が marks the grammatical subject of the verb.

Here, the thing that started is:

• 新しい商品の開発 = the development of a new product

So:

• 新しい商品の開発が始まった
• The development of a new product started

The development is what began, so it is marked by が.

This is a very common question because Japanese often has both an intransitive and a transitive version of a verb.

• 始まる = to begin / to start
  
  • something begins
• 始める = to begin / to start something
  
  • someone starts something

In this sentence:

• 開発が始まった
• the development started

The development itself is beginning, so Japanese uses the intransitive verb 始まる.

If you used 始めた, it would mean someone started it, for example:

• 会社が新しい商品の開発を始めた。
• The company started the development of a new product.

始まった is the past form of 始まる.

• 始まる = starts / will start
• 始まった = started / began

Japanese past tense often covers both:

• started
• has started

depending on context.

So this sentence can feel like:

• The development of a new product started.
• The development of a new product has begun.

Both are reasonable translations in English.

Yes. That whole part acts as one noun phrase:

• 新しい商品 = a new product
• 新しい商品の開発 = development of a new product

Then が marks that whole phrase as the subject of 始まった.

So the sentence structure is basically:

• [新しい商品の開発] が [始まった]
• [The development of a new product] [started]

Japanese does not have articles like a, an, and the.

So 商品 can mean:

• a product
• the product
• sometimes just product(s)

The exact meaning depends on context.

That is why 新しい商品の開発 could be understood in English as:

• the development of a new product
• development of a new product

Usually English translation adds a or the to sound natural.

Possibly in the real-life situation, yes, but not in the grammar of this sentence.

This sentence does not say who started the development. It only says that:

• the development began

Japanese often leaves out information that is understood from context.

So if the speaker wanted to say who started it, they could say something like:

• 会社が新しい商品の開発を始めた。
• The company started the development of a new product.

But in your sentence, the focus is only on the event beginning.

Yes. Normal Japanese writing would be:

新しい商品の開発が始まった。

The spaces in your example are just there to make it easier for learners to see the parts.

Japanese normally does not put spaces between words the way English does.

Here are the standard readings:

• 新しい → あたらしい
• 商品 → しょうひん
• の → の
• 開発 → かいはつ
• が → が
• 始まった → はじまった

So the full sentence is read:

あたらしい しょうひん の かいはつ が はじまった。

Yes, often it can.

• 新しい商品 = a new product
• 新商品 = a new product / new item

So:

• 新しい商品の開発
• 新商品の開発

can both mean development of a new product.

However, there is a slight nuance:

• 新商品 is a compact noun expression often used in business or advertising.
• 新しい商品 is a more straightforward descriptive phrase.

In many contexts, 新商品の開発が始まった would sound very natural too.

The core pattern is:

Noun phrase + が + intransitive verb

In this case:

• 新しい商品の開発 = noun phrase
• が = subject marker
• 始まった = intransitive verb in past tense

So the pattern is:

• [Something] が 始まった
• [Something] started

You can make many similar sentences with this pattern, for example:

• 会議が始まった。 = The meeting started.
• 工事が始まった。 = Construction started.
• 授業が始まった。 = Class started.