Learn Hardcore Japanese: watasi ha hiru ni hon wo yomimasu.

は marks the topic of the sentence. Here it tells us “as for 私 (I), we’re talking about me.” It doesn’t necessarily mark the grammatical subject in the English sense, but rather the theme or what the sentence is about.

Why is に used after 昼?

The particle に marks a specific point in time when an action occurs. 昼に means “at noon” or “in the afternoon.” Without に, 昼 alone would just mean “daytime” or “noon” in a more general sense, without specifying “when.”

What role does を play in 本を読みます?

を is the direct-object marker. It tells us that 本 (“book”) is what’s being acted upon by the verb. 本を読みます therefore means “read a book.”

Why is the verb 読む in the form 読みます?

読みます is the polite, non-past form (often called the -masu form) of 読む. It’s used in everyday conversation when you want to be polite or neutral, covering both present and future actions (“read” or “will read”).

Why does the time expression 昼に come before the object 本を?

Japanese typically follows the order: Topic – Time – Place – Object – Verb (T-T-P-O-V). So time expressions like 昼に (“at noon/afternoon”) precede objects like 本を, and the verb 読みます comes at the very end.

Could you use が instead of は in 私は?

You could, but it changes the nuance. 私が would emphasize “I” as the one who does the action (often when introducing new information or contrasting). 私は simply sets “I” as the topic, neutral and more common when you’re just stating what you do.

Is it okay to drop 私は and just say 昼に本を読みます?

Yes. Japanese is often context-driven. If it’s clear who is doing the action, you can omit the pronoun/topic and simply say 昼に本を読みます (“(I) read a book in the afternoon”).

How would you change the sentence if you read books every day?

You could add 毎日 (every day) before the verb:
私は昼に 毎日 本を読みます。
Literally: “As for me, at noon, every day, I read books.”