kono zisyoapuri ha hontou ni benri de, nihongo no benkyou ni yaku ni tatimasu.

は marks the topic of the sentence. Here, この辞書アプリは means as for this dictionary app or this dictionary app, ....

The sentence is then making comments about that topic:

• it is really convenient
• it is useful for studying Japanese

So は does not mean the app is doing an action; it simply sets up what the sentence is about.

この is a word that must come before a noun. Since 辞書アプリ is a noun, you use この to say this dictionary app.

Compare:

• これ = this
• このアプリ = this app
• この辞書アプリ = this dictionary app

So これ can stand alone, but この must directly modify a noun.

本当に means really or truly and works as an adverb modifying 便利.

• 本当 = truth / real / true
• 本当に = really / truly

In this sentence:

• 本当に便利 = really convenient

This is a very common pattern:

• 静かだ → 静かに
• 本当だ → 本当に

So the に turns it into a form that can modify another word, especially an adjective or verb.

Because 便利で is connecting to the next part of the sentence.

The sentence has two linked ideas:

• this app is really convenient
• it is useful for studying Japanese

便利 is a na-adjective, and its connecting form is で:

• 便利で、役に立ちます = convenient, and useful

If you said 便利です, that would sound more like the sentence is ending there. To connect smoothly to another statement, 便利で is used.

Yes, 便利 is a na-adjective.

The な appears when the adjective directly modifies a noun:

• 便利なアプリ = a convenient app

But in this sentence, 便利 is being used as a predicate:

• このアプリは便利です or in a connecting form:
• このアプリは便利で、...

So there is no な because it is not directly modifying a noun here.

日本語の勉強 means the study of Japanese or studying Japanese.

The particle の connects 日本語 and 勉強, showing what kind of study it is:

• 数学の勉強 = study of math
• 歴史の勉強 = study of history
• 日本語の勉強 = study of Japanese

In natural English, you would usually say studying Japanese, but in Japanese it is very common to use this noun phrase pattern.

That に goes with the expression 役に立つ, which means to be useful for or to be of use in.

So:

• 日本語の勉強に役に立つ = to be useful for Japanese study

This に marks the thing that benefits from the usefulness.

You can think of it like:

• useful for something

Examples:

• 仕事に役に立つ = useful for work
• 旅行に役に立つ = useful for travel

役に立つ is an expression meaning to be useful or to be helpful.

Literally:

• 役 = role / service / usefulness
• に立つ = to stand in / to serve as

But as a fixed expression, 役に立つ simply means to be useful.

In polite form:

• 役に立つ → 役に立ちます

So the whole phrase:

• 日本語の勉強に役に立ちます means
• it is useful for studying Japanese

They are doing different jobs.

1. 本当に

   • this に turns 本当 into an adverb: really

2. 勉強に役に立ちます

   • this に marks what something is useful for

So even though both are written に, they are not the same kind of function.

立ちます is the polite present/future form of 立つ.

Dictionary form:

• 役に立つ

Polite form:

• 役に立ちます

Japanese often uses the polite form in normal explanations, product descriptions, recommendations, and conversation with people you are not close to. Since this sentence sounds like a general statement about an app, 立ちます is very natural.

The comma shows a pause and helps separate the two linked ideas:

• 本当に便利で、
• 日本語の勉強に役に立ちます。

It is not absolutely required in every sentence, but it makes the sentence easier to read. Japanese commas often reflect natural pauses or clarify structure.

No. Normal Japanese would usually be written without spaces:

この辞書アプリは本当に便利で、日本語の勉強に役に立ちます。

The spaces are often added in teaching materials to make the parts of the sentence easier for learners to see. In real Japanese text, you normally would not expect those spaces.