Learn Hardcore Russian: Кто бы ни сидел рядом на семинаре, я концентрируюсь на теме и не отвлекаюсь. - Whoever sits next to me at the seminar, I focus on the topic and don’t get distracted.

Yes, this construction is very close to an English subjunctive / concessive idea like “no matter who is sitting” or “whoever may be sitting”.

кто бы ни сидел literally combines:
- кто – who(ever)
- бы – the particle used to form “subjunctive-like” / hypothetical meanings
- ни – a particle that adds the sense of “no matter / regardless”
- сидел – past tense form of сидеть, used with бы to create that hypothetical/concessive feel

So кто бы ни сидел ≈ whoever may be sitting / no matter who is sitting.

It’s a standard pattern:

кто бы ни пришёл – whoever comes
что бы ни случилось – whatever happens
где бы ни был – wherever (he) may be

Why is сидел in the past tense when we are talking about a general present-time situation?

In Russian, when you combine бы with a past-tense verb, you often get a meaning similar to the subjunctive or hypothetical mood in English, not literal past time.

So:

кто бы ни сидел – grammatically past, but functionally “whoever may be sitting” / “no matter who sits”

Other examples:

если бы он пришёл – if he came / if he were to come
кто бы он ни был – whoever he may be

So the past tense here is not about time; it’s about the mood created with бы.

Why is it ни and not не in кто бы ни сидел?

In concessive, “no matter…”-type constructions, Russian uses ни, not не.

Patterns:

кто бы ни сидел – no matter who is sitting
что бы ни случилось – no matter what happens
где бы ты ни был – no matter where you are
как бы ни было трудно – no matter how hard it is

не is the normal negation (not), but ни in these patterns is more like “ever / at all / no matter”, giving a universal, regardless-of-who/what/where meaning.

Using кто бы не сидел would sound either incorrect or, in some rare contexts, would be interpreted differently and much less idiomatic here.

Why is сидел in masculine singular if кто can refer to a woman too?

Кто is grammatically masculine singular in Russian, regardless of the actual gender of the person. So the verb normally agrees with it in masculine singular:

кто пришёл? – who came? (masc. sg.)
кто бы ни писал – whoever may be writing (masc. sg.)
кто бы ни сидел – whoever may be sitting (masc. sg.)

This does not mean the person must be male; it is just grammatical default agreement. Context decides the actual gender.

What exactly does рядом mean here, and why is there no с кем?

Рядом means “near / next to / beside”. In this sentence:

кто бы ни сидел рядом – whoever is sitting next (to me / to us / to someone, understood from context)

Russian often omits the object when it’s obvious:

Садись рядом. – Sit next to me. (with me understood)
Он в комнате, дети играют рядом. – He is in the room, the kids are playing nearby.

You could add it:

кто бы ни сидел рядом со мной на семинаре… But it’s not necessary if it’s clear you mean “next to me at the seminar.”

Why is it на семинаре and not на семинар?

Both exist, but they mean different things:

на семинар (accusative) – movement towards the seminar:
- я иду на семинар – I’m going to the seminar.
на семинаре (prepositional) – location at the seminar:
- я на семинаре – I am at the seminar.
- кто бы ни сидел рядом на семинаре – whoever is sitting next to me at the seminar.

Here we’re talking about what happens during / at the seminar, not about going there, so на семинаре is correct.

What is the nuance of the present-tense verbs концентрируюсь and не отвлекаюсь here?

They describe a general, habitual behavior, not a single moment:

я концентрируюсь на теме – I concentrate on the topic (as a rule / whenever this situation occurs)
и не отвлекаюсь – and I don’t get distracted (I tend not to; I make it a habit)

Russian present tense can mean:

actual present: сейчас я читаю – I’m reading now
habitual/general: я каждый день читаю – I read every day

In this sentence, combined with кто бы ни сидел…, it clearly means a general, consistent habit: “No matter who sits next to me at the seminar, I stay focused and don’t get distracted.”

Why are концентрируюсь and отвлекаюсь reflexive verbs (ending in -сь)?

The reflexive -сь here doesn’t mean you are doing something “to yourself” in a literal way; in these verbs it has more of a lexical / idiomatic function.

концентрироваться (на чём?) – to concentrate (on something)
- я концентрируюсь на теме – I concentrate on the topic.
отвлекаться (от чего?) – to get distracted (from something)
- я не отвлекаюсь – I don’t get distracted.

Without -ся, the verbs either mean something different or are not used in this meaning:

концентрировать usually needs a direct object: концентрировать силы, внимание – to concentrate one’s forces, attention (on something/someone else).
отвлекать – to distract someone: не отвлекай меня – don’t distract me.

So -сь in концентрируюсь, отвлекаюсь creates “middle voice” meanings: you experience this mental action yourself.

Why is there a comma before я концентрируюсь?

The structure is:

Кто бы ни сидел рядом на семинаре, – subordinate concessive clause
я концентрируюсь на теме и не отвлекаюсь. – main clause

Russian uses a comma to separate a subordinate clause from the main one, especially when the subordinate clause (кто бы ни сидел…) comes first.

This is similar to English:

Whoever sits next to me at the seminar, I focus on the topic and don’t get distracted.

Could you rephrase this sentence in a slightly simpler or more neutral way?

Yes, for a bit more neutral or simpler style, you could say:

Кем бы ни был мой сосед на семинаре, я концентрируюсь на теме и не отвлекаюсь.
(Literally: Whoever my neighbor at the seminar may be…)

or more colloquially:

Не важно, кто сидит рядом на семинаре, я концентрируюсь на теме и не отвлекаюсь.
(It doesn’t matter who sits next to me at the seminar…)

The original with кто бы ни сидел is perfectly natural and somewhat “bookish”/standard, very good for written and careful spoken Russian.

Is there any difference between this and кто сидит рядом на семинаре, я концентрируюсь…?

Yes, there is a big difference in meaning and correctness:

кто бы ни сидел рядом на семинаре… – correct; means “no matter who sits next to me…”
кто сидит рядом на семинаре, я концентрируюсь… – sounds wrong / incomplete.

Without бы ни, кто сидит is just a normal question word or a relative pronoun (“who sits”), not a concessive “whoever / no matter who”.

To express the same idea, you must use one of the concessive patterns:

кто бы ни сидел…
не важно, кто сидит…
кто угодно сидит рядом, а я всё равно концентрируюсь…

What aspect are концентрируюсь and отвлекаюсь, and would perfective forms change the meaning?

Both концентрируюсь (from концентрироваться) and отвлекаюсь (from отвлекаться) are imperfective.

Imperfective here is natural because we’re talking about:

a general habit, a repeated situation
an ongoing process of concentrating / (not) getting distracted

The perfective counterparts:

сконцентрироваться – to manage to concentrate / to get focused (single completed achievement)
отвлечься – to get distracted once (a single event)

Using perfective would change the meaning, for example:

…я сразу сконцентрируюсь на теме и не отвлекусь.
– “I will immediately manage to concentrate on the topic and will not get distracted (even once).”

That sounds more like a promise about one specific future occurrence, not a general rule. In the original sentence, imperfective correctly expresses a stable habit.