wakaranai tango ga ooi node, kyou ha zisyoapuri wo tukaukoto ni simasu.

Because 分からない is modifying the noun 単語.

In Japanese, verbs and adjectives can come directly before a noun to describe it. So:

• 分からない単語 = words that I don’t understand

This works a lot like a relative clause in English, but Japanese does not need words like that, which, or who here.

So the structure is:

• 分からない → don’t understand
• 単語 → words / vocabulary words
• 分からない単語 → words I don’t understand

Because 多い is an adjective, not a verb that takes a direct object.

In Japanese, with 多い (many / a lot), the thing that is numerous is marked with が:

• 単語が多い = there are many words
• more literally: the words are many

So in this sentence:

• 分からない単語が多いので = because there are many words I don’t understand

A native English speaker may expect を because English often thinks in terms of I don’t understand many words, but Japanese builds this part differently.

Both can relate to quantity, but they behave differently.

• 多い is an i-adjective
• たくさん is more like an adverb/noun-like word meaning a lot

So:

• 単語が多い = natural, standard way to say there are many words
• 単語がたくさんある = also natural, but structured differently: there are a lot of words

In your sentence, 多い fits neatly after 単語が.

Both can mean because, but ので usually sounds a bit softer, more explanatory, and slightly more formal than から.

Compare:

• 分からない単語が多いので、今日は辞書アプリを使うことにします。
• 分からない単語が多いから、今日は辞書アプリを使うことにします。

Both are correct.

The nuance:

• ので = gives a reason in a calm, explanatory way
• から = can sound more direct or personal

In this sentence, ので sounds very natural because the speaker is explaining the reason for their decision.

は marks 今日 as the topic: as for today / today, ...

So:

• 今日は = today (as the topic)
• It can also imply a slight contrast, like at least for today or for today in particular

The sentence feels like:

• Because there are many words I don’t understand, as for today, I’ll use a dictionary app.

Using は helps frame the rest of the sentence around today.

〜ことにする is a very common grammar pattern meaning:

• to decide to do
• to make it a rule to do
• sometimes to go with doing

So:

• 使う = use
• 使うこと = the act of using / using
• 使うことにします = I will decide to use it / I’ve decided to use it

In this sentence, it means the speaker is making a decision:

• 今日は辞書アプリを使うことにします。
• Today, I’ll use a dictionary app.
• more literally: I’ll decide on using a dictionary app today.

こと turns the verb phrase into a noun-like idea.

• 使う = to use
• 使うこと = using / the act of using

This nominalized phrase can then be followed by grammar patterns like:

• 〜ことにする = decide to do
• 〜ことになる = it is decided that... / it turns out that...

So こと is needed because にする attaches to the action as a noun-like concept.

This comes from the grammar pattern 〜ことにする.

The にする part often means to decide on or to make it into something. In this pattern, it means:

• make it a decision to do X
• more naturally: decide to do X

So:

• 使うことにします = I decide to use it
• polite form of 使うことにする

This is a fixed expression, so it is best learned as a set pattern:

• dictionary form: 〜ことにする
• polite present/future: 〜ことにします
• past: 〜ことにした
• polite past: 〜ことにしました

They are similar, but not identical.

1. 使います

Simply says I will use it or I use it, depending on context.

• straightforward
• no special emphasis on the decision-making process

2. 使おうと思います

Means I think I’ll use it or I’m thinking of using it.

• sounds like an intention
• a bit softer or less firm

3. 使うことにします

Means I’ve decided to use it.

• emphasizes the decision itself
• often used when choosing among options

So in your sentence, 使うことにします fits well because the speaker is reacting to a situation and deciding what to do.

Because Japanese often omits subjects when they are clear from context.

Here, the sentence naturally implies I:

• (I) don’t understand many words
• (I) will use a dictionary app today

Japanese frequently leaves out 私, あなた, 彼, etc. if the listener can infer them.

So even though English normally needs I, Japanese does not.

Not quite.

分からない

Means:

• don’t understand
• don’t know the meaning
• can’t make sense of

知らない

Means:

• don’t know
• am not familiar with
• have never learned/heard of

With vocabulary, both can appear in some contexts, but the nuance changes.

• 知らない単語 = a word I don’t know
• 分からない単語 = a word I don’t understand

In this sentence, 分からない単語 suggests the speaker is encountering words whose meaning they can’t understand, which matches the idea of using a dictionary app.

Because Japanese often combines nouns directly to make compound nouns.

• 辞書 = dictionary
• アプリ = app
• 辞書アプリ = dictionary app

This is very common in Japanese, especially with modern words and technology-related terms.

You could think of it like English compounds such as:

• phone app
• language app
• dictionary app

Japanese often does this without の.

No. In normal Japanese, this sentence would usually be written without spaces:

分からない単語が多いので、今日は辞書アプリを使うことにします。

Spaces are often added only in teaching materials to make the structure easier for learners to see. So the spaced version is helpful for study, but not standard everyday writing.