nihongo no dorama wo mite iru uti ni, sizenna hyougen ga sukosizutu wakaru you ni narimasita.

Because 日本語 is modifying ドラマ.

• 日本語のドラマ = Japanese-language dramas / dramas in Japanese
• The particle の links nouns and shows that 日本語 describes what kind of drama it is.

So here, 日本語 is not the object of a verb. It is part of the noun phrase 日本語のドラマ.

〜ているうちに means while doing something, as you keep doing something, or in the course of doing something.

So:

• 見ている = am watching / was watching / keep watching
• うちに = while; during the time that; before one realizes it

Together, 見ているうちに gives the idea:

• while I was watching
• as I kept watching
• in the course of watching

It often suggests a gradual or natural change that happens during that process.

Both can be possible in some contexts, but 見ているうちに puts more focus on the ongoing action of watching.

• 見ている emphasizes the continuing state: while watching
• 見るうちに can also mean as one watches, but it may sound a little more general or less focused on the ongoing process

In this sentence, 見ているうちに fits well because the speaker is talking about a change that happened gradually during repeated or continued watching.

うち literally means something like inside or within, but in grammar 〜うちに often means:

• while
• during
• before the situation changes

The に marks the time or point within which something happens.

So 見ているうちに means something like:

• within the time of watching
• more naturally: while watching / as I kept watching

This pattern is very common in Japanese.

Because the verb is 分かる, not 分ける or some other transitive verb.

In Japanese, 分かる usually works with が:

• 表現が分かる = to understand expressions

Even though English says understand something, Japanese treats the thing understood as the grammatical subject of 分かる.

So:

• 自然な表現が分かる = natural expressions become understandable / I can understand natural expressions

This is one of the most important patterns to get used to with 分かる.

自然な表現 means natural expressions, natural ways of saying things, or natural wording.

• 自然な = natural
• 表現 = expression, way of expressing something

In this sentence, it refers to Japanese expressions that sound natural to native speakers, not just grammatically correct textbook Japanese.

少しずつ means little by little, gradually, or bit by bit.

It shows that the change did not happen all at once. The speaker slowly became able to understand more natural Japanese expressions over time.

So it adds a nice sense of gradual progress.

This is the pattern:

• dictionary form + ようになる

It means:

• to come to be able to...
• to start to...
• to reach the point where...

So:

• 分かる = understand
• 分かるようになる = come to understand / become able to understand

And:

• なりました = polite past of なる

So 分かるようになりました means:

• came to understand
• became able to understand

It emphasizes a change over time.

No, the sentence does not use 分かりました. It uses 分かるようになりました, which is different.

Compare:

• 分かりました = I understood / I got it
• 分かるようになりました = I came to understand / I became able to understand

The second one emphasizes development or improvement. That is a much better fit here, because the speaker is talking about gradual learning through watching dramas.

In V-dictionary form + ようになる, よう helps create the meaning it becomes the case that... or one comes to be able to...

So:

• 分かるようになる is not just become understand
• It means become such that one understands

This is a very common Japanese structure for expressing change in ability or condition.

Examples:

• 日本語が話せるようになった = I became able to speak Japanese
• 早く起きられるようになった = I became able to wake up early

So in your sentence, 分かるようになりました shows a change in ability or understanding.

Yes. Japanese often leaves out the subject when it is clear from context.

In English, you would probably say:

• I gradually came to understand natural expressions...

But in Japanese, the I is not necessary here. It is naturally understood from context.

This omission is extremely common in Japanese and does not make the sentence incomplete.

It usually suggests a process over time, often through repeated or continued watching.

Because the second half says:

• 自然な表現が少しずつ分かるようになりました

the whole sentence clearly describes gradual improvement, not a one-time sudden realization.

So the natural interpretation is:

• By watching Japanese dramas over time, I gradually came to understand natural expressions.

In Japanese, nouns often do not show singular or plural clearly.

So 表現 can mean:

• an expression
• expressions

In this sentence, the meaning is naturally understood as expressions in general or more natural ways of speaking, not just one specific phrase.

That is why English often translates it in the plural.

A simple breakdown is:

• 日本語のドラマを = Japanese dramas
  • object marker
• 見ているうちに = while watching / as I kept watching
• 自然な表現が = natural expressions
• 少しずつ = little by little
• 分かるようになりました = came to understand / became able to understand

So the structure is:

While watching Japanese dramas, I gradually came to understand natural expressions.

More literally:

In the course of watching Japanese dramas, natural expressions gradually became understandable to me.