Learn Hardcore Japanese: ani ha ame no hi ni ie de hon wo yomimasu. - My older brother reads a book at home on rainy days.

は is the topic-marking particle. It indicates that “older brother” is the theme or background of the sentence—what we’re talking about. In contrast, が is the subject-marking particle, used to introduce new or focal information. If you said 兄が雨の日に…, it would feel like you’re emphasizing “it is my brother (and not someone else) who…” rather than stating a habitual fact. Habitual or general statements typically use は.

How is 雨の日に formed? What does the の do in 雨の日 and why is に used afterward?

雨の日 uses の to connect two nouns—雨 (“rain”) and 日 (“day”)—literally “rain’s day,” i.e. “rainy day.”
adding に after a time expression marks the point in time when something happens.
So 雨の日に means “on a rainy day” or “on rainy days.”

Why is 家で used here? What’s the difference between で and に when indicating location?

で marks the location where an action takes place. Here, 家で means “at home,” i.e. the place where the reading happens.
に can also mark location but is used for existence (e.g. 家にいる, “to be at home”) or destination (e.g. 家に帰る, “to return home”).

What does the particle を indicate in 本を読みます?

を is the direct-object particle. It shows that 本 (“book”) is the thing being acted upon by the verb 読む (“to read”). Without を, it would be unclear what is being read.

Why is the verb in the -ます form (読みます)? Could we use the plain form 読む instead?

読みます is the polite present/future form of 読む. It’s appropriate in formal or polite speech.
In casual or written Japanese, you can use the plain (dictionary) form: 兄は雨の日に家で本を読む。 Both sentences mean the same habitual action, but the politeness level differs.

Is 読みます a present-progressive (“is reading”) or simple present (“reads”)? How would you say “He is reading a book right now”?

読みます describes a simple present or habitual action, not an ongoing one.
To express “is reading” (an action in progress), you use the ている form:
“兄は今、家で本を読んでいます。”

Why does the verb come at the end of the sentence? Is word order important in Japanese?

Japanese follows a Subject–Object–Verb (SOV) order. Modifiers like topics, time phrases, places, and objects all precede the verb. The verb 読みます must come last, so you know when the action is introduced and completed.

Does 兄 automatically mean “my older brother”? How do you specify whose brother it is?

In context, 兄 generally refers to the speaker’s older brother. Japanese often omits explicit possessives when the relationship is clear.
To clarify or talk about someone else’s brother, add a possessive: 私の兄 (my older brother), 彼の兄 (his older brother), etc.