kono jishoapuri wa hontou ni benri de, nihongo no benkyou ni yaku ni tachimasu.

topic particle

Breakdown of kono jishoapuri wa hontou ni benri de, nihongo no benkyou ni yaku ni tachimasu.

• 日に本ほん語ご: Japanese (language)
• の: possessive case particle

Questions & Answers about kono jishoapuri wa hontou ni benri de, nihongo no benkyou ni yaku ni tachimasu.

Why is は used after この辞書アプリ?

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.

Why is it この辞書アプリ instead of これ辞書アプリ?

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.

What does 本当に do here, and why does 本当 become 本当に?

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.