Learn Hardcore Russian: Сегодня днём я решил немного поучиться языку в библиотеке. - This afternoon I decided to study the language for a bit in the library.

сегодня днём ≈ “this afternoon” / “earlier today (in the day, not in the evening or morning)”.
It usually refers to the middle part of the day (roughly late morning to late afternoon), contrasting with:
- сегодня утром – this morning
- сегодня вечером – this evening

So it’s very close to “this afternoon”, but a bit broader: it’s the general daytime period of today, not a strict clock time.

What case is днём, and why does it look like that?

Днём is the instrumental singular form of день (day).

Form:

Nominative: день (day)
Instrumental: днём

The instrumental is often used adverbially to express time when:

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

So сегодня днём is literally “today, in the daytime”, where днём is instrumental used in a time sense.

Could I change the word order and say Я решил сегодня днём немного поучиться языку в библиотеке? Does it change the meaning?

Yes, you can. Russian word order is relatively flexible.

Both:

Сегодня днём я решил немного поучиться языку в библиотеке.
Я решил сегодня днём немного поучиться языку в библиотеке.

are correct and natural.

Differences:

Initial Сегодня днём slightly emphasizes when it happened.
Initial Я slightly emphasizes the subject (“I”), but in this simple sentence the difference is very subtle.

In normal conversation, both versions would be understood the same way. The original version just foregrounds the time a bit more.

Why is it решил and not some other form? What does this tell us about the subject?

Решил is:

past tense
masculine singular
perfective
of the verb решить (to decide).

In Russian, past tense verbs agree with the subject in gender and number:

я решил – I decided (said by a man)
я решила – I decided (said by a woman)
мы решили – we decided
они решили – they decided

So решил implies the speaker is male (or that the subject is a masculine noun).
If a woman says this sentence, it would be:

Сегодня днём я решила немного поучиться языку в библиотеке.

What’s the difference between решить учиться and решить поучиться?

Both are grammatically correct, but they have different nuances:

решить учиться – “to decide to study (in general, as an ongoing thing)”
- Suggests a more long-term or general decision.
- Example: Я решил учиться русскому языку. – I decided to study Russian (as a course of study).
решить поучиться – “to decide to study for a while / study a bit”
- По- adds the sense of doing something for a short time / a bit.
- Perfective поучиться focuses on a limited, completed session of studying.

In your sentence, решил немного поучиться means:

He decided to study for a bit, not to start some serious, long-term program.

What exactly does поучиться mean, and how is it different from учиться?

Base verb: учиться – to study, to learn (generally, to be engaged in learning).

Adding the по- prefix:

поучиться – to study for a while, for some time, or a bit.

Nuance:

учиться – a neutral, ongoing process: “I am studying / I study”.
поучиться – focus on having a short session of studying, often one occasion.

Examples:

Я люблю учиться языкам. – I like studying languages (in general).
Я хочу поучиться часок. – I want to study for an hour or so.

In your sentence, поучиться highlights the limited, one-time nature of the study session.

Why do we have both немного and поучиться together? Doesn’t that repeat the idea of “a little”?

They do partly overlap in meaning, but together they sound natural and just reinforce the idea of “a bit”.

поучиться already suggests “to study for a while / a bit”.
немного means “a little, not much”.

Немного поучиться = to study a little bit, not for too long and not too intensively.

Using both is common in Russian for emphasis:

немного поспать – sleep a little
немного поработать – work a little

So it’s not considered redundant; it simply makes the “small amount” idea stronger and more natural.

Why is it поучиться языку (dative) and not поучиться язык (accusative)?

Because the verb учиться / поучиться takes the dative case for what you are learning.

Patterns:

учить (что?) язык – to study/learn a language (direct object in accusative)
- учить русский язык – to study Russian (language)
учиться (чему?) языку – to study/learn a language (dative)

So:

поучиться языку is correct (what to learn? языку, dative).
поучиться язык would be incorrect.

Compare:

Я учу русский язык. – I am learning Russian.
Я учусь русскому языку. – I am learning Russian.

Meaning is similar, but the grammar pattern (and nuance) is different.

Can I specify which language and say поучиться русскому языку? Does the case change?

Yes, you can say поучиться русскому языку, and the case pattern stays the same: dative.

русский язык – nominative (dictionary form)
русскому языку – dative (after учиться / поучиться)

Examples:

Я хочу поучиться русскому языку. – I want to study (some) Russian.
Он решил немного поучиться французскому языку. – He decided to study some French.

Both adjective (русскому) and noun (языку) must agree in case (dative).

Why is it в библиотеке and not в библиотеку?

The difference is location vs direction:

в библиотеке – in the library (where something takes place)
- Uses the prepositional case (библиотеке).
в библиотеку – to the library (movement towards)
- Uses the accusative case (библиотеку) with the preposition в.

In your sentence:

поучиться языку в библиотеке – to study the language in the library (location).

If you wanted to emphasize the movement, you’d say:

Сегодня днём я решил пойти в библиотеку поучиться языку.
– This afternoon I decided to go to the library to study the language.

How do we know it means “in the library” and not “in a library”, since Russian has no articles?

Russian has no articles (no “a/an” or “the”), so библиотеке by itself can mean in a library or in the library.

Which one it feels like in English depends entirely on context:

If you have a specific library in mind (e.g., your usual library), English will often translate it as “in the library”.
If you are speaking more generally, you might say “in a library”.

In Russian, the form в библиотеке doesn’t change; only the English translation chooses a or the based on context.

Is a comma needed after Сегодня днём? Could I write Сегодня днём, я решил…?

Normally, no comma is used here:

Сегодня днём я решил немного поучиться языку в библиотеке.

Short, simple adverbials of time at the beginning of a sentence (like Сегодня днём, Вчера, Утром) usually come without a comma.

Сегодня днём, я решил… would be considered incorrect or at least nonstandard in modern Russian punctuation.