mori no naka ni yami ga atta.

A natural breakdown is:

• 森 = forest
• の = linking particle, here making 森 modify 中
• 中 = inside / middle
• に = particle marking the place where something exists
• 闇 = darkness
• が = subject marker
• あった = past tense of ある, to exist / to be

So the structure is basically:

[forest’s inside] + in + [darkness] + existed

In more natural English grammar terms, it is:

In the forest, there was darkness or There was darkness in the forest.

の connects nouns. It often works like of, or like turning one noun into a modifier for another noun.

So:

• 森 = forest
• 中 = inside

Together, 森の中 means the inside of the forest, or more naturally, in the forest / inside the forest.

This is not possession in the English sense. It is just a very common noun-linking pattern in Japanese:

• 家の中 = inside the house
• 箱の中 = inside the box
• 森の中 = inside the forest

This is a very common question.

With ある and いる, に marks the place where something exists.

So:

• 森の中に闇があった = There was darkness in the forest

By contrast, で usually marks the place where an action happens:

• 森の中で歩いた = walked in the forest
• 森の中で何かが起こった = something happened in the forest

Since darkness existed is not really an action being performed, に is the normal choice.

が is natural here because the sentence is presenting or identifying what existed in that place.

Pattern:

• 場所に + ものがある
• place + thing exists

So 闇があった means there was darkness.

If you changed it to 闇はあった, it would feel different. は would add contrast or make darkness the topic, as if you were saying something like:

• As for darkness, it was there
• Darkness, at least, was there

That is possible in some contexts, but が is the neutral, standard choice for introducing an existing thing.

Japanese uses:

• ある for things that are not considered animate
• いる for people and animals

闇 = darkness, which is not animate, so it takes ある.

Past tense:

• ある → あった

Examples:

• 木があった = there was a tree
• 犬がいた = there was a dog
• 人がいた = there was a person

So 闇があった is correct because darkness is treated as a non-living thing.

あった is the plain past form of ある.

• dictionary form: ある
• past plain form: あった

It means was / existed

This is plain style, which is the basic non-polite form often used in narration, fiction, casual speech, and internal thoughts.

The polite version would be:

• 森の中に闇がありました。

That is grammatically fine, but it sounds more polite and less like narrative prose.

Both are possible.

Japanese word order is more flexible than English because particles show each word’s role.

But the usual order often puts the location before the thing that exists:

• 森の中に闇があった

This feels very natural because it first sets the scene, then tells you what was there.

If you say:

• 闇が森の中にあった

that is still grammatical, but it places more focus on 闇 first. It can sound a bit more marked depending on context.

So the original order is a very natural storytelling order:

1. set the location
2. introduce what existed there

Here, 中 is read なか.

So the full reading is:

• 森 = もり
• の
• 中 = なか
• に
• 闇 = やみ
• が
• あった

Altogether:

もり の なか に やみ が あった。

It can be both, but 闇 often sounds more literary, dramatic, or heavy than the more everyday word 暗さ or just 暗い.

So 闇 can mean:

• literal darkness
• gloom
• figurative darkness
• something ominous or emotionally heavy

In a sentence like this, especially in a story, 闇 often gives a strong atmospheric effect. It sounds more evocative than a plain description of something being dark.

Yes, very much.

Japanese often uses this pattern:

• 場所に何かがある = there is something in a place
• 場所に誰かがいる = someone is in a place

So this sentence follows a very common existence pattern:

• 森の中に = in the forest
• 闇があった = there was darkness

That is why it maps neatly to English There was darkness in the forest, even though the Japanese structure is not literally word-for-word the same.

They are almost certainly just there for learners.

Normal Japanese writing usually does not put spaces between words:

森の中に闇があった。

Textbooks, study materials, and beginner-friendly examples sometimes add spaces to make the structure easier to see. The actual sentence in ordinary Japanese would be written without them.