Новая грамматическая тема: неопределённые местоимения с суффиксом «-нибудь».

In Russian, неопределённые местоимения are pronouns that refer to someone/something in a vague, not clearly identified way.
They answer questions like “who?”, “what?”, “where?” etc., but without specifying exactly:

• кто-нибудь – someone / anyone
• что-нибудь – something / anything
• где-нибудь – somewhere / anywhere
• когда-нибудь – sometime / ever

They are “indefinite” because the person/thing/place is not precisely defined.

-нибудь is treated as a suffix-like particle that attaches to interrogative words to form indefinite pronouns and adverbs.

• It is not written as a separate word.
• It is always joined with a hyphen: кто-нибудь, что-нибудь, где-нибудь, когда-нибудь.
• Grammatically, dictionaries often describe it as a postfix / particle, but for learners it’s fine to think of it as a “suffix” added to the pronoun.

The bare forms:

• кто? – who?
• что? – what?
• где? – where?
• когда? – when?

With -нибудь, they stop being questions and become indefinite:

• кто-нибудь – someone / anyone
• что-нибудь – something / anything
• где-нибудь – somewhere / anywhere
• когда-нибудь – sometime / ever

Typical nuance of -нибудь:

• “some (it doesn’t matter which)”,
• “any at all”,
• “random, unspecified”.

Both form indefinite pronouns, but the nuance is different.

-то usually means “some particular but unknown person/thing” – the speaker vaguely has someone/something in mind:

• Кто-то звонил. – Someone (a particular person, I don’t know who) called.

-нибудь often means “any at all / random, it doesn’t matter which”:

• Позовите кого-нибудь. – Call someone (any person will do).
• Дайте мне какую-нибудь ручку. – Give me some pen / any pen.

Very roughly:

• -то ≈ “some (specific but unknown)”
• -нибудь ≈ “some / any (it doesn’t matter which; random)”

It is used mainly with interrogative pronouns and adverbs, such as:

• кто → кто-нибудь – someone / anyone
• что → что-нибудь – something / anything
• какой → какой-нибудь – some (kind of)
• который → который-нибудь – some (particular) one (less common)
• где → где-нибудь – somewhere / anywhere
• куда → куда-нибудь – (to) somewhere / anywhere
• когда → когда-нибудь – sometime / ever
• как → как-нибудь – somehow / in some way

You do not attach -нибудь to personal pronouns like я, ты, он, мы etc. There is no я-нибудь or он-нибудь.

Yes, the pronoun still declines; the base part changes, but -нибудь stays the same.

For кто-нибудь:

• Nominative: кто-нибудь – someone, anyone
• Genitive: кого-нибудь – of someone / anyone
• Dative: кому-нибудь – to someone / anyone
• Accusative: кого-нибудь – someone / anyone (as object)
• Instrumental: кем-нибудь – by/with someone / anyone
• Prepositional: о ком-нибудь – about someone / anyone

Similarly, что-нибудь:

• чего-нибудь, чему-нибудь, чем-нибудь, о чём-нибудь, etc.

In normal pronunciation, -нибудь is usually unstressed.
The main stress stays on the original interrogative word:

• кто́-нибудь
• что́-нибудь
• како́й-нибудь
• когда́-нибудь
• где́-нибудь

So you generally keep the original stress and pronounce -нибудь lightly after it: [kto-NI-byt’], [shto-NI-byt’], etc., but with the stress still perceived on кто́ / что́ / како́й / когда́ / где́.

Because of the preposition с (“with”).

• When с means “with” in the sense of “together with, accompanied by”, it requires the instrumental case in Russian.
• суффикс is masculine; its instrumental singular form is суффиксом.

So:

• с суффиксом – with the suffix
• (compare: с другом – with a friend, с книгой – with a book)

The main noun is тема, which is:

• feminine,
• singular,
• in the nominative case here.

Adjectives must agree with the noun in gender, number, and case, so:

• Feminine singular nominative of новый → новая
• Feminine singular nominative of грамматический → грамматическая

Therefore: Новая грамматическая тема.

In Russian, a colon is used much like in English to introduce:

• an explanation,
• a list,
• or a more specific description of what was mentioned before.

Here:

• Новая грамматическая тема: – “New grammar topic:”
• неопределённые местоимения с суффиксом «-нибудь» – the specific topic itself.

So the colon simply introduces the detailed name of the new topic.

Yes. Grammatically, forms with -нибудь are classified as indefinite pronouns (or indefinite adverbs if it’s something like где-нибудь or когда-нибудь).

For example:

• кто – interrogative pronoun (“who?”)
• кто-нибудь – indefinite pronoun (“someone / anyone”)

Even though they’re built from an interrogative base + a particle (-нибудь), they function as a single pronoun in sentences.

It depends on the context, but general tendencies:

• In questions and many conditionals, they often correspond to “any-” words:

  • Ты что-нибудь ел? – Did you eat anything?
  • Если увидишь кого-нибудь, скажи мне. – If you see anyone, tell me.

• In neutral requests or offers, they often look like “something / someone”:

  • Принеси что-нибудь поесть. – Bring me something to eat.
  • Позови кого-нибудь. – Call someone.

• When emphasizing “it doesn’t matter which / random”, English often uses “any”:

  • Возьми какую-нибудь книгу. – Take any book (it doesn’t matter which one).

No. In Russian, negation works differently from English. Russian tends to use negative pronouns together with не (double negation is normal).

For people, instead of кто-нибудь you use никто / никого:

• Incorrect: Я не знаю кто-нибудь.
• Correct: Я никого не знаю. – I don’t know anyone. / I know nobody.

Similarly for things:

• Incorrect: Я не вижу что-нибудь.
• Correct: Я ничего не вижу. – I don’t see anything. / I see nothing.

кто-нибудь / что-нибудь are used:

• in questions: Ты кого-нибудь знаешь? – Do you know anyone?
• in conditionals: Если кто-нибудь придёт… – If anyone comes…
• in requests, suggestions, etc., without a main negation.

Their stylistic feel:

• -нибудь – generally neutral, everyday language.
• -то – also neutral, very common in speech.
• -либо – tends to sound more formal, bookish, or technical (common in documents, legal language, logical statements).

Examples:

• кто-нибудь – someone/anyone (neutral)
• кто-то – someone (neutral, often a specific but unknown person)
• кто-либо – someone / anyone (formal, written style)

In the topic name неопределённые местоимения с суффиксом «-нибудь», the focus is just on one particular type; it doesn’t imply any special formality level by itself.