Learn Hardcore Russian: Мне трудно концентрироваться вечером, когда я устал. - It is hard for me to concentrate in the evening when I am tired.

So Мне трудно концентрироваться literally means:
“To me it is difficult to concentrate” → natural English: “It’s hard for me to concentrate.”

You do not say Я трудно концентрируюсь; that sounds ungrammatical or very odd.
The pattern is:

Мне / Тебе / Ему / Ей / Нам / Вам / Им + трудно / легко + infinitive
- Мне трудно вставать рано. – It’s hard for me to get up early.
- Ей легко говорить по-русски. – It’s easy for her to speak Russian.

What exactly is трудно here? Is it an adjective like “hard,” or something else?

In this sentence трудно is a predicative adverb, a special type that is used in impersonal constructions:

Мне трудно… – It’s hard for me…
Ему легко… – It’s easy for him…
Нам скучно… – We’re bored / It’s boring for us…

It doesn’t agree with any noun in gender, number, or case; it just describes the general state of things. It’s similar to English “it’s hard / it’s easy / it’s boring”.

So grammatically, трудно = “hard (to do something)” in the sense of “it is hard”, not describing a noun directly.

Why is концентрироваться reflexive (with -ся) instead of just концентрировать?

Russian distinguishes between:

концентрировать (что?) – to concentrate something (transitive)
- концентрировать внимание на задаче – to concentrate attention on a task.
концентрироваться (на чём?) – to concentrate (oneself), focus (intransitive, reflexive)
- концентрироваться на задаче – to concentrate on a task.

In Мне трудно концентрироваться, the meaning is “It’s hard for me to concentrate (myself)”, so Russian uses the reflexive verb.

Using non‑reflexive концентрировать by itself (without a direct object) would be incorrect here.

Why is it вечером and not в вечер or в вечере?

Вечером is the instrumental singular of вечер and is commonly used by itself to mean “in the evening / at night (in the evening time)”.

Russian often uses the instrumental case without a preposition to express “in/at (time of day)”:

утром – in the morning
днём – in the daytime / during the day
вечером – in the evening
ночью – at night

So:

Мне трудно концентрироваться вечером. – It’s hard for me to concentrate in the evening.

You do not say в вечер in this meaning. You might see к вечеру (“towards evening”), but that’s a different idea.

Why is the first part using an infinitive (концентрироваться), but the second part is past tense (я устал)?

You have two different structures/situations:

Мне трудно концентрироваться…
- This uses the infinitive because it describes the general activity: to concentrate.
- Pattern: кому + трудно / легко + infinitive.
когда я устал
- This is a finite clause with subject я and verb устал (past tense, perfective).
- It describes a state resulting from becoming tired: when I am (have become) tired.

So the sentence combines:

General difficulty with an activity (infinitive), and
The condition/state under which it happens (past-tense finite verb).

Russian often uses past tense forms like устал in a general statement about when something is hard if the focus is on the resulting state (tired) rather than the ongoing process of getting tired.

What is устал exactly? Is it a verb, an adjective, and does it change for gender?

Устал is the short masculine form of the adjective усталый (“tired”), historically linked to the verb устать (“to get tired”). In practice, it behaves like a short-form adjective meaning “(has) become tired / is tired.”

It changes with gender and number of the subject:

Я устал. – I am tired. (male speaker)
Я устала. – I am tired. (female speaker)
Мы устали. – We are tired.
Оно устало. – It is tired. (neuter)

So for a female speaker, your original sentence would be:

Мне трудно концентрироваться вечером, когда я устала.

Could I drop я and just say когда устал?

Yes, in informal spoken Russian, people often drop the subject pronoun when it’s obvious:

Мне трудно концентрироваться вечером, когда устал.

This is understood as “when I’m tired”, especially if it’s you speaking about yourself.

However:

In formal writing or when clarity matters, it’s better to keep я:
когда я устал / когда я устала.
If the subject might be ambiguous, omitting я can cause confusion.

So with friends it’s fine; for clear, textbook Russian, keep я.

Why is there a comma before когда?

Because когда я устал is a subordinate clause of time (“when I’m tired”). In Russian:

Subordinate clauses introduced by conjunctions like когда, потому что, если, хотя are separated by a comma from the main clause.

Structure here:

Main clause: Мне трудно концентрироваться вечером
Subordinate clause (time): когда я устал

So you write:

Мне трудно концентрироваться вечером, когда я устал.

Can I change the word order? For example, say Когда я устал, мне трудно концентрироваться вечером?

Yes. Russian word order is relatively flexible, and all of these are natural:

Мне трудно концентрироваться вечером, когда я устал.
Когда я устал, мне трудно концентрироваться вечером.
Мне вечером трудно концентрироваться, когда я устал.
Когда я устал, вечером мне трудно концентрироваться.

The differences are mostly about emphasis and rhythm:

Starting with Когда я устал… highlights the condition first.
Moving вечером closer to трудно or концентрироваться slightly emphasizes “in the evening”.

Grammatically, they’re all fine.

What’s the difference between трудно, тяжело, and сложно here?

All three could appear in similar sentences, but they have different nuances:

трудно – “hard, difficult” in a broad, neutral sense.
- Мне трудно концентрироваться вечером…
тяжело – literally “heavy”; often implies emotional or physical burden / strain.
- Мне тяжело концентрироваться вечером… – It is very hard / burdensome for me; feels heavier, more “suffering”.
сложно – from сложный (“complex”); often implies complexity or complication, sometimes mental difficulty.
- Мне сложно концентрироваться вечером… – It’s hard because it’s mentally demanding or complicated to focus.

In everyday speech, трудно is the default neutral choice.

Could I say Мне трудно сконцентрироваться вечером instead of концентрироваться? What’s the difference?

Yes, but the aspect and meaning change:

концентрироваться – imperfective: general, repeated, ongoing process.
- Мне трудно концентрироваться вечером – In general, it’s hard for me to concentrate (as an ongoing activity) in the evenings.
сконцентрироваться – perfective: single, completed act.
- Мне трудно сконцентрироваться вечером – It’s hard for me to manage to concentrate / to get myself focused (on a particular occasion) in the evening.

So:

Talking about a general habit/regular situation → use концентрироваться.
Talking about one specific time or success/failure to “get focused” → сконцентрироваться is more natural.

How would I negate this sentence properly?

There are two main patterns, with different meanings:

Negating the difficulty:
- Мне не трудно концентрироваться вечером, когда я устал.
  – It’s not hard for me to concentrate in the evening when I’m tired.
Negating the action of concentrating:
- Мне трудно не концентрироваться вечером, когда я устал.
  – It’s hard for me *not to concentrate in the evening when I’m tired.*
  (i.e., I tend to focus anyway, and stopping myself is difficult.)

In your original meaning (it is hard to concentrate), you would say:

Мне трудно концентрироваться вечером, когда я устал.
Мне очень трудно концентрироваться вечером, когда я устал. (to intensify)

Is this sentence specifically masculine? How would a woman say it?

The only gender-sensitive part is устал. Everything else is gender‑neutral.

Masculine speaker:
Мне трудно концентрироваться вечером, когда я устал.
Feminine speaker:
Мне трудно концентрироваться вечером, когда я устала.
Plural (“we”):
Нам трудно концентрироваться вечером, когда мы устали.

So you just change устал according to the subject’s gender/number.