Learn Hardcore Japanese: zisyoapuri ha nihongo no benkyou ni totemo yaku ni tatimasu. - The dictionary app is very useful for studying Japanese.

辞書アプリは = As for dictionary apps… / Speaking of dictionary apps…
Then the sentence tells us something about that topic: they are very useful for studying Japanese.

If you said 辞書アプリが日本語の勉強にとても役に立ちます, it would sound more like:

It is dictionary apps that are very useful for studying Japanese (slight emphasis/contrast on 辞書アプリ).

So:

は: sets up the topic, neutral statement about it.
が: marks the subject, often with a sense of new information or contrast.

In this kind of general statement, は is the most natural choice.

Why is it 日本語の勉強 and not 日本語を勉強?

日本語の勉強 is a noun phrase:

日本語 = Japanese (language)
勉強 = study, studying (noun)
日本語の勉強 = the study of Japanese / Japanese study

Here, the sentence structure is:

日本語の勉強 (noun phrase)
に (particle)
役に立ちます (is useful)

So it literally reads:

Dictionary apps are very useful *for the study of Japanese.*

If you used 日本語を勉強, you’d need to continue it as a verb phrase, e.g.:

日本語を勉強するのにとても役に立ちます。
They are very useful for studying Japanese (for the act of studying Japanese).

Both are correct Japanese, but in your original sentence they chose the simpler noun phrase 日本語の勉強.

What does the particle に do in 日本語の勉強に?

Here, に marks the target or purpose – roughly “for” in English.

日本語の勉強に役に立ちます。
= [They] are useful *for the study of Japanese.*

This に answers the question: useful for what?

Similar patterns:

旅行に便利です。 – It’s convenient for travel.
健康にいいです。 – It’s good for your health.

So X に役に立つ means to be useful for X.

Could we use で instead of に in 日本語の勉強に?

No, で would be unnatural here.

に in 日本語の勉強に役に立つ means “for” (the purpose / target of usefulness).
で usually marks a place, means, or situation (e.g. at, by, with, in).

For example:

学校で勉強します。 – I study at school.
辞書アプリで単語を調べます。 – I look up words with a dictionary app.

You can’t say 日本語の勉強で役に立つ to mean useful for studying Japanese. The natural choice is に.

What exactly does 役に立ちます mean, and how is it formed?

役に立ちます is the polite form of the verb 役に立つ, an idiomatic expression meaning:

to be useful, to be helpful, to be of use

Breakdown:

役 (やく) – role, use, function
に – marks the role / function
立つ (たつ) – to stand

Originally, 役に立つ is like “to stand in (its) role,” i.e., to serve its function, to be useful.

So:

辞書アプリは日本語の勉強にとても役に立ちます。
= Dictionary apps are very useful for studying Japanese.

Dictionary form: 役に立つ
Polite present: 役に立ちます
Polite past: 役に立ちました (was useful)

Why is there a に inside 役に立ちます? Is it the same に as in 日本語の勉強に?

They are two different uses of に:

日本語の勉強に – “for studying Japanese”
- に marks the target of usefulness.
役に立つ – a fixed expression where 役に is part of an idiom.
- Historically “stand in one’s role,” but learners usually just remember 役に立つ as a single chunk meaning to be useful.

So the whole structure is:

日本語の勉強に (for studying Japanese)
役に立ちます (is useful)

You don’t need to analyze the second に every time; just memorize 役に立つ as one unit.

What’s the difference between 役に立つ and 便利, since both can be translated as “useful”?

They overlap, but the nuance is slightly different.

役に立つ – helpful / serves a purpose / actually helps you achieve something.
- Strong focus on practical usefulness.
便利（べんり） – convenient / handy / makes things easier.
- Focus on ease, convenience, efficiency.

In many contexts you can use either:

辞書アプリは日本語の勉強にとても役に立ちます。
辞書アプリは日本語の勉強にとても便利です。

Both are fine. The first emphasizes that it really helps your study. The second emphasizes that it makes studying Japanese more convenient.

Why is it 立ちます and not just 立つ?

立つ is the dictionary/plain form.
立ちます is the polite -ます form.

役に立つ – plain (used in dictionaries, casual speech, written explanations)
役に立ちます – polite (used with teachers, strangers, in public writing, etc.)

The sentence is in polite style overall:

辞書アプリは日本語の勉強にとても役に立ちます。

If you were speaking casually to a friend, you might say:

辞書アプリは日本語の勉強にめっちゃ役に立つよ。

Does 立ちます here mean present or future? Is it “are useful” or “will be useful”?

Japanese present tense (〜ます / 〜る) covers:

present habits / general truths
and future statements

So 役に立ちます can be:

are useful (general statement)
will be useful (future-oriented)

In context, this sentence means something like:

Dictionary apps are (generally) very useful for studying Japanese.
or
Dictionary apps will be very useful when you study Japanese.

The nuance depends on context, but grammatically it’s the same form.

What is とても doing in this sentence? Are there other words I can use instead?

とても is an adverb meaning very. It strengthens 役に立ちます:

役に立ちます – is useful
とても役に立ちます – is very useful

Other common intensifiers:

すごく役に立ちます。 – really / amazingly useful (more casual)
本当に役に立ちます。 – truly / really useful
かなり役に立ちます。 – quite useful

とても is neutral and polite, so it fits nicely here.

Why is it just 辞書アプリ and not 辞書のアプリ?

辞書アプリ is a compound noun: 辞書 (dictionary) + アプリ (app). Japanese often combines nouns directly like this:

英語アプリ – English(-learning) app
天気アプリ – weather app
ゲームアプリ – game app

辞書アプリ = dictionary app – an app whose type/function is “dictionary.”
If you said 辞書のアプリ, it sounds more like “an app that belongs to a dictionary” or “an app of a dictionary,” which is not the normal way to name app categories. The direct compound is more natural.

What is 勉強 here? Isn’t 勉強する the verb “to study”?

勉強 by itself is a noun: study, studying.
勉強する is the verb: to study.

In your sentence:

日本語の勉強 = the study of Japanese (noun phrase)
That noun phrase is followed by に役に立ちます (is useful for).

So the structure is:

辞書アプリは / 日本語の勉強に / とても / 役に立ちます。
Dictionary apps / for the study of Japanese / very / are useful.

You could rephrase it with the verb:

辞書アプリは日本語を勉強するのにとても役に立ちます。
Dictionary apps are very useful for studying Japanese.

Both are natural; one treats 勉強 as a noun, the other as part of the verb 勉強する.

Is the word order fixed, or can I move とても or 日本語の勉強に around?

Japanese word order is relatively flexible as long as:

The verb 役に立ちます stays at the end.
Particles stay attached to the right words.

Your original sentence:

辞書アプリは日本語の勉強にとても役に立ちます。

You could say:

辞書アプリはとても日本語の勉強に役に立ちます。
日本語の勉強に辞書アプリはとても役に立ちます。

All are understandable. The most natural and smooth for beginners is usually the original word order:

[topic] は [for what] に [degree word] [verb]
辞書アプリは日本語の勉強にとても役に立ちます。