Learn Hardcore Russian: Мне непривычно сидеть прямо, но я знаю, что хорошая осанка важна для здоровья. - It is unusual for me to sit up straight, but I know that good posture is important for health.

Мне непривычно сидеть прямо literally: To me it is unusual to sit up straight.
Structure: [dative person] + [adverb/adjective in neuter] + infinitive
This pattern is very common for describing states, feelings, or reactions:
- Мне трудно говорить по-русски. – It’s hard for me to speak Russian.
- Ему скучно одиному. – He is bored (it’s boring for him).
Я не привык сидеть прямо. – I am not used to sitting up straight.
This is more direct, focusing on я (I) as the subject with a personal verb привык (got used to / am used to).

In the original sentence, Мне непривычно sounds a bit more impersonal and neutral, like describing a state rather than emphasizing “I” as an active subject.

What exactly is непривычно grammatically? Is it an adjective or an adverb?

In this sentence непривычно is an adverb (or you can think of it as a predicative form), derived from the adjective привычный (habitual, usual).

Adjective: привычный – привычная поза (a usual posture)
Adverb: привычно – Он действует привычно. (He acts in a usual way.)
With negation: непривычно – not usual, unfamiliar:
- Мне непривычно вставать так рано.

So here: Мне непривычно сидеть прямо = It is unfamiliar/unusual for me to sit up straight.

Why is непривычно written as one word, not не привычно?

Непривычно is normally written together because:

It is a single concept: unusual / not customary.
It comes from непривычный (unusual), which is also one word.

You write не separately mainly when you want to strongly contrast or when the word doesn’t form a normal single concept with не:

Он сидит не прямо, а криво. – He sits not straight but crooked. (strong contrast)
Это не привычно, а странно. – That’s not habitual, but strange. (two different qualities)

In the sentence given, there is no contrast, so непривычно is one word.

Why is мне in the dative case here?

Мне is dative because Russian often uses the dative to mark the experiencer of a state or feeling.

Pattern: [Dative of person] + [adjective/adverb (neuter)]
Examples:

Мне холодно. – I am cold. (It’s cold to me.)
Ей скучно. – She is bored. (It’s boring to her.)
Нам трудно. – It’s hard for us.

So Мне непривычно сидеть прямо literally: To me, it is unusual to sit straight.
The person who experiences the state gets the dative.

Why is the verb сидеть in the infinitive, and why right after непривычно?

The infinitive сидеть is used to express what is unusual for the speaker.

Common pattern:

Мне трудно читать по-русски. – It’s hard for me to read in Russian.
Ему приятно работать дома. – It’s pleasant for him to work at home.
Мне непривычно сидеть прямо. – It’s unusual for me to sit up straight.

So:

непривычно = describes the general feeling/state
сидеть (infinitive) = the action that causes that feeling

The infinitive often follows immediately after such words because together they form one semantic unit.

What is the nuance of сидеть прямо? Is it the same as “sit straight up” or “sit straight”?

Сидеть прямо normally means to sit upright / to sit up straight, as opposed to slouching or leaning.

прямо literally = straight
- идти прямо – to go straight (ahead)
- смотреть прямо – to look straight (directly)

Here it describes the way of sitting:

сидеть прямо – to keep your back straight, not slouching

You might also hear:

сидеть ровно – also “sit straight / evenly,” often similar in this context
сидеть с прямой спиной – to sit with a straight back
But сидеть прямо is the simplest and very common phrase for “sit up straight.”

Why do we have хорошая осанка важна, not хорошая осанка важная?

Важна is a short-form adjective (краткая форма), while важная is the full form.

Хорошая осанка важна для здоровья.
= Good posture is important for health.
(важна = acts like the English is important, a predicate.)
важная осанка would mean important posture as an attribute before a noun, which sounds strange here.

Compare:

Эта тема важна. – This topic is important. (short form, predicate)
Это важная тема. – This is an important topic. (full form, attribute before noun)

In our sentence, we are saying what good posture is (it is important), so we must use the short form: важна.

How do short-form adjectives like важна work in general?

Short-form adjectives:

Agree only in gender and number, not in case:
- важен (masc.), важна (fem.), важно (neut.), важны (pl.)
Are used mainly as predicates (after a noun/pronoun), often where English uses is/are + adjective:
- План важен. – The plan is important.
- Работа трудна. – The work is difficult.
- Это возможно. – This is possible.

Full-form adjectives (e.g. важный) are used:

Before nouns: важная проблема (an important problem)
Sometimes also as predicates, but often with a subtle stylistic/semantic difference.

In our sentence, осанка is feminine, so: осанка важна.

Why is there no word for “is” in хорошая осанка важна для здоровья?

Russian normally omits the present-tense verb “to be” (быть) in such sentences.

Structure:

[subject] + [short/long adjective or noun]
- Он врач. – He is a doctor.
- Книга интересная. – The book is interesting.
- Хорошая осанка важна. – Good posture is important.

If you put есть (the present form of быть) in this kind of sentence, it usually sounds:

Emphatic, contrastive, or unnatural in everyday speech:
- Хорошая осанка есть важна… – could sound like heavy emphasis: “Good posture really is important…” (and might feel a bit odd in normal conversation).

So the natural version drops “is” entirely.

What case is здоровья in, and why do we use для здоровья?

Здоровья is in the genitive singular.
The preposition для (“for”) always takes the genitive:

для ребёнка – for a child
для работы – for work
для здоровья – for (one’s) health

So:

хорошая осанка важна для здоровья
= good posture is important for health / for one’s health

You wouldn’t say для здоровье, because здоровье must change form after для.

Could you explain the overall word order? Could we say Осанка хорошая важна для здоровья or Важна хорошая осанка?

The neutral, most natural order here is:

Хорошая осанка важна для здоровья.
- хорошая осанка – subject (with adjective before the noun)
- важна – predicate (short-form adjective)
- для здоровья – prepositional phrase, extra information

Other orders are possible but change focus or may sound unnatural:

Важна хорошая осанка для здоровья. – Puts more emphasis on важна (“It is important that good posture…”) or on важна, slightly more stylistic/poetic.
Осанка хорошая важна для здоровья. – This sounds incorrect or at least very unnatural; two adjectives (“осанка хорошая важна”) are competing, and the structure is unclear.

So, for normal speech/writing, Хорошая осанка важна для здоровья is the right word order.

Is there any special reason for the comma before но and before что?

Yes; both commas are required by standard Russian punctuation rules.

Comma before но
Но (“but”) is a coordinating conjunction joining two clauses:
- Мне непривычно сидеть прямо, но я знаю… Each side has its own verb (сидеть, знаю), so we have two clauses, and Russian puts a comma before но.
Comma before что
Что here is a subordinating conjunction introducing an object clause:
- я знаю, что хорошая осанка важна для здоровья Literally: “I know that good posture is important for health.”
  In Russian, such что-clauses are almost always separated by a comma.