Learn Hardcore Russian: Самодисциплина важна, но иногда мне не хватает дисциплины вечером. - Self‑discipline is important, but sometimes I lack discipline in the evening.

Самодисциплина важна literally: “Self-discipline important (is).”
- самодисциплина – feminine noun (nominative singular)
- важна – short-form feminine adjective agreeing with самодисциплина

You could also say:

Самодисциплина является важной. – more formal and heavy, literally “Self-discipline is (constitutes) something important.”
Самодисциплина важная. – sounds incomplete by itself, like “Self-discipline is an important … (something)”. You normally need a noun after it, for example:
- Самодисциплина — важная черта характера. – “Self-discipline is an important character trait.”

For simple “X is important” statements, [noun in nominative] + [short-form adjective] is the most natural choice:
Самодисциплина важна. / Книга интересна. / Работа трудна.

Why is it мне не хватает дисциплины, literally “to me it is not enough discipline”? How does this construction work?

Мне не хватает дисциплины is a very common idiomatic way to say “I lack discipline” / “I don’t have enough discipline.”

The structure:

мне – dative case, “to me”
не хватает – 3rd person singular of хватать, here used impersonally: “there is not enough / I lack”
дисциплины – genitive singular, “(of) discipline”

So the idea is: “For me, there is not enough discipline.”

This follows a general pattern:

Мне хватает времени. – I have enough time.
Мне не хватает денег. – I don’t have enough money / I lack money.
Нам не хватает опыта. – We lack experience.

Instead of the English “I lack X”, Russian likes this impersonal “to me (there) is not enough X” construction with the dative + (не) хватает + genitive.

Why is дисциплины in the genitive case and not дисциплина in the nominative?

Хватать / не хватать almost always governs the genitive when it means “to be enough / not to be enough (of something).”

Мне хватает воды. – I have enough water.
Ему не хватает терпения. – He lacks patience.
Им не хватает знаний. – They lack knowledge.

So:

не хватает (чего?) дисциплины – genitive (дисциплины), because it answers the “of what?” question.

Semantically, it’s similar to English “there isn’t enough of discipline,” which also suggests that idea of “of something,” corresponding to the genitive.

Why is дисциплины singular, not plural like дисциплин?

Here дисциплина is used in an abstract, uncountable sense: “discipline” as a quality, not specific “disciplines” or “subjects.”

Мне не хватает дисциплины. – I lack discipline (as a personal quality).
- genitive singular of this abstract noun.

Дисциплин (genitive plural) would mean “of the disciplines / of the school subjects,” e.g.:

Не хватает дисциплин по математике. – There aren’t enough math-related courses.

In your sentence, we’re talking about personal self-control, so the singular abstract дисциплины is correct.

What is the difference between самодисциплина and дисциплина here? Why use both?

самодисциплина – specifically self-discipline: your own ability to control yourself, stick to plans, etc.
дисциплина – more general “discipline”: order, organization, being disciplined.

In this sentence:

Самодисциплина важна – a general statement about the importance of the ability (self-discipline as a concept).
мне не хватает дисциплины вечером – personal confession: I don’t show enough discipline (I’m not disciplined enough) in the evening.

Using самодисциплина twice would also be possible:

Самодисциплина важна, но иногда мне не хватает самодисциплины вечером.

But native speakers often vary wording; дисциплина here sounds slightly more everyday and personal than repeating the abstract term самодисциплина.

Could I replace дисциплины with a pronoun like её (“it”) so I don’t repeat the word?

Grammatically, yes:

Самодисциплина важна, но иногда мне её не хватает вечером.

This is correct and quite natural in conversation.

Nuance:

дисциплины – keeps the word explicit; stylistically very neutral.
её – sounds a bit more colloquial and context-dependent (you must already clearly know that её refers to самодисциплина).

Both are fine; using the noun again is normal in Russian and doesn’t feel as repetitive as in English.

Why is вечером used, not something like в вечер or в вечере?

Вечером is an adverbial form meaning “in the evening / at night (in the sense of evening time).”

It is historically an instrumental case used adverbially. Russian often uses such forms to mean “at a certain time”:

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

You normally don’t say в вечер or в вечере in this sense. Instead, you use:

вечером – “in the evening” (habitual / general)
по вечерам – “in the evenings” (regularly, on multiple evenings)

So мне не хватает дисциплины вечером = “I lack discipline in the evening (as a general pattern).”

Is there any difference if I say Но иногда вечером мне не хватает дисциплины instead of но иногда мне не хватает дисциплины вечером?

Both are correct; the difference is mainly in rhythm and slight emphasis:

…иногда мне не хватает дисциплины вечером.
- More neutral; the time вечером comes at the end as additional information.
…иногда вечером мне не хватает дисциплины.
- Slightly more emphasis on evening as the time when this happens: “Sometimes, in the evening, I lack discipline.”

Word order in Russian is flexible and often used to highlight what is important. Putting вечером earlier can make “evening” sound a bit more contrastive or prominent, but the core meaning is the same.

What’s the difference between не хватает and не хватит? Could I use не хватит here?

хватает – present tense, imperfective: “is enough / is not enough (as a general fact or habit).”
хватит – future tense, perfective: “will be enough / will not be enough (in a specific situation).”

In your sentence:

иногда мне не хватает дисциплины вечером – habitual/general pattern: sometimes, in the evenings, I (generally) lack discipline.

You would not normally say иногда мне не хватит дисциплины вечером in this context; that would sound like:

“Sometimes (in the future), there won’t be enough discipline for me in the evening (for some specific task).”

не хватит is used when talking about whether something will be enough in a particular upcoming situation:

Мне не хватит сил это сделать. – I won’t have enough strength to do it.
Им не хватит времени на проект. – They won’t have enough time for the project.

For regular, repeated or general lack, (не) хватает is the right choice.

Could I say У меня нет дисциплины вечером instead of мне не хватает дисциплины вечером?

You can, but it’s a bit stronger and slightly different in nuance.

Мне не хватает дисциплины вечером. – “I don’t have enough discipline in the evening / I lack discipline (to some degree).”
У меня нет дисциплины вечером. – “I have no discipline in the evening (at all).”

So:

не хватает – suggests insufficiency, not necessarily zero.
нет – more absolute, sounds like “zero discipline.”

In everyday speech people might exaggerate with у меня нет дисциплины вечером, but literally it is more categorical than мне не хватает дисциплины вечером.

What is the overall register of this sentence? Is it formal, neutral, or informal?

The sentence is neutral and can be used in most contexts:

Appropriate in casual conversation:
Самодисциплина важна, но иногда мне не хватает дисциплины вечером.
Also fine in semi-formal writing (a blog post, an essay about productivity, etc.).

If you wanted to sound more formal/academic, you might expand and use more abstract wording, but as it stands, this sentence is standard, neutral Russian.