Learn Hardcore Japanese: watasi ha heya no naka de hon wo yomimasu.

In this sentence, 私 is also the subject in meaning (the one who reads), but grammatically は is marking it as the topic: the thing the rest of the sentence is giving information about.

In other sentences, the grammatical subject can sometimes be different from (or hidden behind) the topic, which is why Japanese teachers insist は = topic marker, が = subject marker.

What does の do in 部屋の中?

の is a particle that usually shows a possessive or attributive relationship, similar to "of" or the English possessive 's.

部屋 = room
中 = inside, middle
部屋の中 = the inside of the room / the room’s inside

So の here connects 部屋 and 中, making "inside (of) the room".

Why is で used after 部屋の中, and what does it mean here?

で is the location-of-action particle. It marks where an action takes place.

部屋の中で = in the room / inside the room (as the place where the reading happens)

Contrast this with:

に: often marks existence, arrival, or direction of movement
- 部屋の中にいる = (I) am in the room.
- 部屋の中に入る = (I) go into the room.
で: marks where an action is performed
- 部屋の中で本を読む = (I) read a book in the room.

So で is used because reading is an action done at that location.

What is the difference between 部屋で and 部屋の中で? Can I just say 部屋で?

You can say 部屋で本を読みます, and it’s perfectly natural.

The nuance:

部屋で = in the room, neutral, simple statement of location.
部屋の中で = inside the room, with a slight extra feeling of "inside-ness" or "within the interior of the room".

Often in everyday speech, people just say 部屋で, unless they want to emphasize "inside" (for example, contrasting with "outside the room").

What does を do in 本を読みます?

を marks the direct object of the verb: the thing the action is done to.

本 = book / books
本を = book(s) as the object
読みます = (I) read

So 本を読みます = (I) read a book / I read books.

In Japanese, word order is flexible, but を always marks what is being acted on by the verb.

Why is the verb 読みます at the end of the sentence? Can I put it earlier like in English?

Japanese is typically SOV: Subject – Object – Verb.

私 (subject / topic)
部屋の中で (location)
本を (object)
読みます (verb)

The verb almost always comes at the end of the clause. You cannot move 読みます to the middle the way English does. Something like:

✗ 私は読みます部屋の中で本を

is incorrect. You can move phrases like 部屋の中で or 本を, but the verb stays at the end. For example:

私は本を部屋の中で読みます。
本を私は部屋の中で読みます。

These sound a bit different in emphasis, but all keep the verb last.

Is 私 necessary? Can I just say 部屋の中で本を読みます?

You can absolutely drop 私. In fact, that’s very natural in Japanese.

部屋の中で本を読みます。

This would normally be understood as "I read (a) book(s) in the room" if the context makes it clear we are talking about you. Japanese often omits pronouns (I, you, he, she, they) when they’re obvious from context.

You keep 私 when:

you want to emphasize "I (and not someone else)", or
the subject is not obvious and you’re clarifying.

Why is it 読みます and not some other form like 読みません or 読んでいます? What exactly does 読みます mean in terms of tense/aspect?

読みます is the polite, non-past form of the verb 読む ("to read").

Dictionary form: 読む
Polite non-past: 読みます

Non-past in Japanese covers both present and future:

I read books (as a habit).
I will read a book.

Context tells you which.

For comparison:

読みません = polite negative non-past ("don’t / won’t read")
読みました = polite past ("read" – completed)
読んでいます = am reading / is reading (ongoing action right now or a continuing state/habit, depending on context)

In your sentence, 読みます is just the neutral polite form: "read / will read".

How is 読みます related to the dictionary form 読む? Why does the stem change?

The verb 読む is a 五段 (godan) verb. To make the polite -ます form, you change the final む to み and add ます:

読む → 読み
- ます → 読みます

This pattern applies to many godan verbs:

飲む → 飲みます (to drink)
書く → 書きます (to write)
話す → 話します (to speak)

So 読みます is simply the polite version of 読む.

Why does English translation say "a book" or "books" when Japanese just has 本 with no article or number?

Japanese does not use articles like "a / an / the". 本 alone can be “a book”, “the book”, or “books”, depending on context.

To be more specific, Japanese can optionally add:

a counter:
- 一冊の本 = one book
- 二冊の本 = two books
a demonstrative:
- この本 = this book
- その本 = that book

In your sentence, 本を読みます is simply “read book(s)”, and the translator has to choose "a book" or "books" based on context or natural English phrasing.

Could I say 部屋の中に本を読みます instead of 部屋の中で本を読みます?

No, 部屋の中に本を読みます is unnatural / incorrect for “read a book in the room.”

に with places is usually for location of existence, arrival, or direction:
- 部屋の中にいる = be in the room
- 部屋の中に入る = go into the room
で is for location where an action happens:
- 部屋の中で本を読む = read a book in the room

So for actions like 読む, 食べる, 勉強する, you normally mark the place with で, not に.

How would I make this sentence negative or past tense while keeping the same basic structure?

Starting from:

私は部屋の中で本を読みます。
- I read / will read a book in the room.

You can change 読みます like this:

Negative (don’t / won’t read)
- 私は部屋の中で本を読みません。
- I don’t / won’t read a book in the room.
Past (read – completed)
- 私は部屋の中で本を読みました。
- I read a book in the room.
Past negative (didn’t read)
- 私は部屋の中で本を読みませんでした。
- I didn’t read a book in the room.

The rest of the sentence (私, 部屋の中で, 本を) stays the same; you just change the form of the verb.