Learn Hardcore Japanese: mainiti nihongo wo benkyousurukoto ha yoi syuukan desu. - Studying Japanese every day is a good habit.

こと is a nominalizer: it turns the verb phrase 勉強する (“to study”) into a noun phrase “the act of studying.” By doing this, you can attach particles like は and use the copula です to make statements about that action.

Can I use の instead of こと here?

Yes. In casual speech you can say: • 毎日日本語を勉強するのは良い習慣です。
However, こと sounds more formal or written. Also, with certain expressions (e.g. rules, general statements), こと is preferred over の.

Why is を used after 日本語?

Because 日本語 is the direct object of the verb 勉強する, so you mark it with the object particle を:
日本語を勉強する → “to study Japanese.”

Why does 勉強すること take は and not が?

は marks the topic of the sentence. Here the topic is “the act of studying Japanese every day.”
If you used が, it would feel like you’re introducing new information or emphasizing the subject:
• 毎日日本語を勉強することが良い習慣です。
That’s grammatical but more neutral or explanatory. は makes it a general statement.

Could I say 日本語を毎日勉強することは良い習慣です instead of putting 毎日 first?

Absolutely. Japanese word order is flexible for time expressions and objects:
• 毎日日本語を勉強することは良い習慣です。
• 日本語を毎日勉強することは良い習慣です。
Both mean the same, but putting 毎日 first emphasizes the frequency.

Where is the subject of this sentence?

There is no explicit subject because Japanese often omits it when it’s understood. Here it’s a general statement (“It is a good habit…”), so no need for “I” or “you.”

Why is the copula です used at the end?

Because 習慣 is a noun, you need a copula to complete the sentence politely. Without です, it would sound incomplete in a polite context.

Why do we use 良い instead of the informal いい?

Both 良い and いい mean “good.”
• いい is the colloquial pronunciation.
• 良い is the kanji form, often used in writing or more formal speech.
In this sentence, 良い習慣 looks more formal and fits the polite level.