kono mondai ha bunpou ga hukuzatu desu ga, reibun wo nikai yomeba wakarimasu.

They are doing different jobs.

• この問題は = as for this problem / regarding this problem
  Here, は marks the overall topic.
• 文法が複雑です = the grammar is complicated
  Here, が marks the subject inside that comment.

So the structure is basically:

• As for this problem, the grammar is complicated.

This kind of topic + smaller subject inside the topic is very common in Japanese.

Yes, が can be a subject marker, but here it is a different が.

In 複雑ですが, the が means but / although and connects the two parts of the sentence:

• 文法が複雑ですが = the grammar is complicated, but...

So this sentence uses が in two different ways:

• 文法が → subject marker
• 複雑ですが → connecting but

This is very normal in Japanese.

Because 複雑 is a na-adjective.

With na-adjectives:

• before a noun: use な
  
  • 複雑な文法 = complicated grammar
• at the end of a sentence: use です
  
  • 文法が複雑です = the grammar is complicated

So:

• 複雑なです is incorrect
• 複雑です is correct here

読めば is the -eba conditional form of 読む.

• 読む = to read
• 読めば = if (someone) reads / when (someone) reads

So:

• 例文を二回読めば分かります
  = If you read the example sentence twice, you’ll understand.

A useful warning: although 読めば contains め, it is not the potential form.

Compare:

• 読めば = if you read
• 読める = can read

二回 means two times / twice and works adverbially here.

• 例文を = the thing being read
• 二回 = twice
• 読めば = if you read

So the phrase means:

• if you read the example sentence twice

Japanese often places time/frequency expressions like 二回 before the verb.

Also, 二回 is read にかい.

分かります is the natural everyday verb for understand in many situations.

• 分かります = understand / become clear
• 理解します = understand, comprehend in a somewhat more formal or analytical sense

In learner materials and normal conversation, 分かる is extremely common.

So 読めば分かります sounds very natural:
If you read it twice, you’ll understand.

Because 分かる is usually treated as an intransitive verb in Japanese.

English says I understand it, with a direct object.
Japanese usually expresses the idea differently:

• それが分かります = I understand that / That is understandable

In this sentence, the thing understood is omitted because it is obvious from context. It is basically:

• 例文を二回読めば、（この問題のことが）分かります。

So the idea is not literally understand it with a direct object, but more like it becomes understandable / you’ll get it.

Yes, in natural English it is usually understood as you.

Japanese often omits subjects when they are clear from context. So:

• 例文を二回読めば分かります

naturally means something like:

• If you read the example sentence twice, you’ll understand.

It could also be interpreted more generally as:

• If one reads the example sentence twice, one can understand it.

But in learning materials, instructions, or explanations, you is the most natural reading.

You could say この問題の文法 if you want to explicitly say the grammar of this problem.

But Japanese often prefers a topic-comment structure instead:

• この問題は文法が複雑です
• literally: As for this problem, the grammar is complicated

This sounds very natural and is a common way to describe a feature of something.

So these are similar in meaning:

• この問題は文法が複雑です
• この問題の文法は複雑です

The first one is especially natural in ordinary Japanese.

By itself, 例文 does not clearly mark singular or plural.

It can mean:

• an example sentence
• example sentences

Japanese often leaves number unspecified unless it matters.

In this sentence, because of the context and the verb phrase 二回読めば, many learners will naturally understand it as the example sentence or the example text being referred to in context. But Japanese itself does not force the singular/plural distinction here.

It can mean either, depending on context.

In study/exam contexts, 問題 often means:

• question
• exercise
• problem

So この問題 could be:

• this question
• this exercise
• this problem

All are possible depending on what kind of material this sentence comes from.