Learn Hardcore Russian: В выходные я просто читаю книгу дома.

Habitual meaning – “on weekends / on the weekend in general”:
- В выходные я просто читаю книгу дома.
  → “On weekends I just read a book at home.” (this is the most natural default reading of your sentence)
Specific future/next weekend – often in context:
- В выходные я просто читаю книгу дома, никуда не поеду.
  → “This weekend I’m just going to stay home and read a book; I’m not going anywhere.”

So you need extra context or intonation to know whether the speaker is talking about their general habit or a specific upcoming weekend.

What is the difference between в выходные and на выходных?

Both are very common and often interchangeable, but there are nuances:

в выходные
- Slightly more neutral/standard.
- Tends to sound a bit more like “on the weekend (days)” as time points.
- Often used for habitual actions:
  - В выходные я поздно встаю. – “On weekends I get up late.”
на выходных
- Feels a bit more colloquial/conversational.
- Very common when referring to a specific weekend (recent or upcoming):
  - Что ты делал на выходных? – “What did you do this weekend?”
  - На выходных поеду к родителям. – “I’ll go to my parents’ this weekend.”

In your example, you could also say:

На выходных я просто читаю книгу дома.

It would still sound natural, just slightly more colloquial.

Which case is выходные in after в here, and why doesn’t it change form?

After в (meaning “into / on (a time period)”) we use the accusative case for time expressions.

в + accusative is common with days and time periods:
- в понедельник – on Monday
- в июле – in July
- в выходные – on the weekend / at the weekend

выходные is a plural adjective used as a noun. Its nominative plural and accusative plural look the same:

Nominative plural: выходные (дни)
Accusative plural (inanimate): выходные

So you don’t see a change, but grammatically it’s accusative plural.

Does я просто читаю here mean “I am reading” (right now) or “I (usually) read”?

Russian present tense of an imperfective verb (like читать) can mean both:

Right now / currently – “I am reading”:
- Сейчас я читаю книгу. – “I am reading a book now.”
Habit / repeated action – “I (usually) read”:
- Я читаю книги по вечерам. – “I read books in the evenings.”

In your sentence, because we have the time phrase в выходные, the natural interpretation is habitual:

В выходные я просто читаю книгу дома.
→ “On weekends I just (tend to) read a book at home.”

Context could very rarely make it mean “This coming weekend, I’m just going to read a book at home,” but the default understanding is a regular habit.

What does просто add here? Is it like English “just”?

Yes, просто here corresponds closely to English “just / simply” and has a similar “nothing special” feeling:

В выходные я просто читаю книгу дома.
→ “On weekends I just read a book at home (I don’t do anything special).”

Nuances of просто:

просто = simply, just (no complication, nothing fancy):
- Я просто хочу отдохнуть. – “I just want to rest.”
- Это просто пример. – “This is just a/an simple example.”

Word order can change emphasis:

Я просто читаю книгу дома. – The whole activity is nothing special; “I just read a book at home.”
Я читаю просто книгу дома. – Slightly odd; might sound like “I read just a book at home (nothing more than a book).” Much less natural in this context.

So your original word order is the most natural.

Why is it книгу and not книга?

книга is a feminine noun.

In the sentence, книгу is the direct object of the verb читаю (I read), so it must be in the accusative case:

Nominative (dictionary form): книга – “book” (subject)
- Книга лежит на столе. – “The book is on the table.”
Accusative (direct object): книгу – “(a/the) book” (object)
- Я читаю книгу. – “I am reading a book.”

So читаю что? – книгу. That’s why it changes to книгу.

Does книгу here mean “a whole book every weekend” or just “I read (some) book”?

книгу in Russian does not automatically mean “one complete book” each time. It’s more like “a book” as an object you are engaged with:

It can mean you’re reading some book (maybe the same one over several weekends).
Russian often uses a singular noun for an ongoing activity, not a completed unit.

If you wanted to emphasize finishing the whole book, Russians might add something:

В выходные я дочитываю книгу. – “On the weekend I (finally) finish the book.”
За выходные я прочитаю книгу. – “Over the weekend I will read (finish) the book.”

Your sentence just says: on weekends you’re occupied with reading a book at home, without focusing on whether you finish it.

Why is it дома and not в доме? What’s the difference?

Both are related to дом (“house / home”), but they’re used differently:

дома (stress on до́ма) = “at home” (location, not moving):
- Я дома. – “I’m at home.”
- Я читаю дома. – “I read at home.”
в доме = “in the house / inside the building” (more literal, physical location):
- В доме холодно. – “It’s cold in the house.”
- Он в доме, а не на улице. – “He is in the house, not outside.”

In your sentence, you’re describing what you do at home in general, so дома is the natural choice:

В выходные я просто читаю книгу дома. – “…I just read a book at home.”

If you said в доме, it would sound like you are emphasizing the inside of some particular house, which is much less typical here.

Can I say Я просто буду читать книгу дома instead? What’s the difference?

You can, but it changes the meaning:

Я просто читаю книгу дома.
- Present tense, imperfective.
- With в выходные, usually habitual:
  → “On weekends I just read a book at home.” (what I generally do)
Я просто буду читать книгу дома.
- Future tense, imperfective (буду + читать).
- Describes what you will be doing over some future period:
  → “I will (just) be reading a book at home.”
- Often used for plans or a specific future weekend:
  - В выходные я просто буду читать книгу дома.
    → “This weekend I’ll just be reading a book at home.”

So:

Use читаю for habits or schedules (and sometimes fixed future plans with a time expression).
Use буду читать when you explicitly want a future progressive meaning: what you’ll be in the process of doing.

Is there any article-like word in Russian for “a/the book” in читаю книгу?

Russian has no articles (no direct equivalents of a, an, the).

The noun книгу on its own can correspond to:

“a book”
“the book”

Which one is meant depends on context, not on a special word:

Я читаю книгу.
- Could be “I’m reading a book” (some book, not specified).
- Or “I’m reading the book” (a specific one you both know about).

In your sentence, without more context, книгу is naturally understood like English “a book” or “some book”.