Learn Hardcore Japanese: watasi no syuukan ha asa ni rekisi no hon wo yomukoto desu. - My habit is to read a history book in the morning.

In Japanese, 習慣 (“habit”) is a noun. When you want to specify whose habit it is, you use the possessive particle の. So 私の習慣 means “my habit.” If you said 私は習慣です, you would literally be saying “I am a habit,” which doesn’t make sense in Japanese.

Why is 習慣 marked with は here? Could we use が instead?

The particle は marks the topic of the sentence—“As for my habit….” We’re introducing 習慣 as what we’re going to talk about. Using が would make 習慣 the grammatical subject focusing on new information, but the nuance changes. With は, we’re defining or describing 私の習慣 rather than simply pointing it out.

Why do we need 読むこと instead of just 読む?

After は and before です, we need a noun or noun phrase as the predicate. A bare verb like 読む is not a noun, so it can’t directly pair with です. By adding こと to the dictionary form 読む, we nominalize the action (“the act of reading”), making 読むこと a noun phrase that fits with です.

What’s the difference between using こと and の to nominalize a verb?

Both こと and の can turn verbs into nouns:

こと is more formal or neutral, commonly used in writing and polite speech.
の is more colloquial and often preferred in casual conversation.
In polite sentences ending with です/だ, こと is generally the standard choice.

Why is 朝 followed by the particle に?

The particle に marks a specific point in time when an action occurs. 朝に means “in the morning.” Without に, 朝 could still imply morning, but adding に makes it more precise: “at that time (in the morning).”

Why do we say 歴史の本 instead of 歴史本?

Japanese uses の to link nouns when forming modifiers: 歴史の本 literally means “a book of history” (i.e. a history book). Although compound nouns like 歴史本 might sometimes appear, the NOUN + の + NOUN pattern is far more common and natural for this kind of description.

Can we reorder the sentence? For example: 私は朝に歴史の本を読むことが習慣です. Is that okay?

Yes. Japanese word order is flexible because particles show each word’s role. Your version—私は朝に歴史の本を読むことが習慣です—is grammatically correct and means essentially the same thing. The nuance shifts slightly: your version foregrounds 私 (“I”), whereas the original starts by introducing 私の習慣 (“my habit”).

Why is there a final です, and could we use だ instead?

です is the polite copula used in formal or polite contexts. It attaches to nominal predicates like 読むこと to complete the sentence politely. In casual speech, you could replace です with the plain copula だ, yielding:
私の習慣は朝に歴史の本を読むことだ。
However, that sounds more direct and less polite.