Learn Hardcore Russian: Мне нужно время, чтобы понять это правило.

Saying я нужно время is ungrammatical because нужно here does not agree with я; it’s not an adjective describing me, but a predicate meaning is necessary.

You could say:

Я нуждаюсь во времени. – I am in need of time.

This is grammatically correct but sounds bookish or formal, and is much less common in everyday speech. The мне нужно / мне надо pattern is far more natural in conversation.

What is the difference between нужно, надо, and нужен / нужна / нужны in this kind of sentence?

All of these can relate to necessity or need, but they’re used somewhat differently.

нужно (impersonal, as in the sentence)
- Мне нужно время. – I need time. / I require time.
  Very common, slightly more neutral/formal than надо but still everyday speech.
надо (impersonal)
- Мне надо время. – I need time.
  Almost the same meaning as нужно here; it’s very common and slightly more colloquial.
нужен / нужна / нужно / нужны (agreeing with the noun)
These are short-form adjectives that agree with what is needed, not with the person:
- Мне нужен карандаш. – I need a pencil. (карандаш – masc.)
- Мне нужна помощь. – I need help. (помощь – fem.)
- Мне нужно время. – I need time. (время – neuter)
- Мне нужны деньги. – I need money. (деньги – plural)
In Мне нужно время, нужно both:
- functions as “it is necessary”,
- and formally agrees in gender (neuter) with время, the needed thing.

In practice, for this sentence:

Мне нужно время and Мне надо время are both natural.
You would not say Я нужно время or Я надо время.

Why is нужно in the neuter form? Is it agreeing with время?

Yes, in this structure нужно is the short neuter form of the adjective нужный and agrees with время:

время – neuter singular
→ нужно – neuter singular short form

Compare:

Мне нужен учебник. – I need a textbook. (учебник – masc.)
Мне нужна ручка. – I need a pen. (ручка – fem.)
Мне нужно время. – I need time. (время – neut.)
Мне нужны деньги. – I need money. (деньги – plural.)

However, there is also an impersonal “нужно” used very broadly, where speakers don’t always strictly follow agreement, especially in casual speech. You may hear Мне нужно деньги in everyday language, though the “correct” agreeing form is Мне нужны деньги.

In your sentence, the standard agreeing form and the impersonal pattern coincide: Мне нужно время is both grammatical and natural.

Why is время in this form and not something like времени?

Время is an irregular neuter noun. Its forms include:

Nominative singular: время
Accusative singular (for inanimate nouns): время (same as nominative)
Genitive singular: времени
Dative singular: времени, etc.

In Мне нужно время:

время is the direct object of “nужно” (the thing that is needed), so it stands in the accusative case.
For inanimate neuter nouns, nominative and accusative are identical. That’s why you see время, not времени.

Времени would appear if you say, for example:

У меня мало времени. – I have little time. (genitive case after мало)

What exactly does чтобы mean here, and why is it followed by an infinitive (понять)?

In this sentence, чтобы introduces a purpose clause:

Мне нужно время, чтобы понять это правило.
→ I need time in order to understand this rule.

Here:

чтобы ≈ in order to / so as to
It is followed by the infinitive понять, because the person needing time (мне) is the same person who will understand the rule.

The pattern is:

(кому) + нужно/надо + (что)
- чтобы + infinitive

Examples:

Мне нужно время, чтобы отдохнуть. – I need time to rest.
Ему нужно время, чтобы всё сделать. – He needs time to do everything.

So here чтобы понять = to understand in the sense of purpose.

Could we say чтобы я понял это правило instead of чтобы понять это правило? What’s the difference?

You can say чтобы я понял это правило, but the nuance and usage are different.

Чтобы понять это правило (infinitive)
- Most natural here.
- Used when the logical subject of both parts is the same:
  - The one who needs time (мне) is also the one who will understand (понять).
- Meaning: I need time in order to understand this rule.
Чтобы я понял это правило (finite verb, subjunctive)
- Literally: so that I might understand this rule.
- This construction is more typical when there is a clearer subject of intention or action in the main clause, often someone else:
  - Он объяснил ещё раз, чтобы я понял это правило.
    He explained once more so that I would understand this rule.
- In Мне нужно время, чтобы я понял это правило, it sounds awkward and redundant, because мне already tells us it’s I who needs the time and will understand.

So, in your original sentence, чтобы + infinitive is the natural choice.

Can I change the word order to Чтобы понять это правило, мне нужно время? Does it change the meaning?

Yes, you can, and it is fully correct:

Чтобы понять это правило, мне нужно время.

The meaning is the same: I need time in order to understand this rule.

Differences:

Original: Мне нужно время, чтобы понять это правило.
- Starts by stating the need (I need time), then explains the purpose.
Reordered: Чтобы понять это правило, мне нужно время.
- Starts by emphasizing the purpose (in order to understand this rule), then states the necessity.

Both are natural. The choice is mostly about emphasis and style, not grammar.

Why is it это правило and not этот правило?

The demonstrative “this” in Russian must agree with the noun’s gender, number, and case.

For правило:

правило is neuter singular.
The nominative/accusative neuter form of “this” is это.

So:

это правило – this rule (neuter)
If the noun were masculine: этот стол – this table
Feminine: эта книга – this book
Plural: эти правила – these rules

Since правило is neuter, это правило is the correct combination.
Этот правило is incorrect because этот is masculine, but правило is neuter.

Is это правило in the nominative or accusative case in this sentence?

Grammatically, это правило is in the accusative case, because it’s the direct object of the verb понять:

понять (что?) это правило – to understand what? this rule.

However, for inanimate neuter nouns, the nominative and accusative singular forms are identical:

Nominative: это правило
Accusative: это правило

So while the form looks nominative, its function in the sentence is accusative.

Could I omit это and just say …чтобы понять правило? Would that change the meaning?

Yes, you can say:

Мне нужно время, чтобы понять правило.

Both versions are grammatical; the difference is nuance:

понять это правило – to understand this rule
- Refers to a specific rule already known from the context (for example, the one you’re currently studying).
понять правило – to understand a/the rule
- Can sound more general or less specific. It could mean “the rule in question”, but without the demonstrative “this”.

In many real contexts, both would be understood as “this particular rule”, but это makes that specificity explicit.

Why is понять (perfective) used instead of понимать (imperfective)?

Russian aspect is very important here.

понять – perfective; focuses on the result, the moment of achieving understanding (to come to understand, to grasp).
понимать – imperfective; focuses on the *ongoing process or repeated action (to be understanding, to understand in general).

When you say:

Мне нужно время, чтобы понять это правило.

you are talking about needing time to reach the result of understanding this rule. This is a one-time achievement, so the perfective is natural.

Using понимать:

Мне нужно время, чтобы понимать это правило.

sounds odd, because it suggests you need time in order to be in a state of understanding it (continuously), which is not how Russians normally phrase this idea.

In purpose clauses like this, чтобы + perfective infinitive is very common when the focus is on achieving a result:

Нам нужно поговорить, чтобы решить проблему. – We need to talk in order to solve the problem.
Мне нужно время, чтобы выучить слова. – I need time to learn (memorize) the words.

Can I drop мне and say Нужно время, чтобы понять это правило? What would that mean?

Yes, you can say:

Нужно время, чтобы понять это правило.

This is also correct. The meaning shifts slightly:

Мне нужно время… – clearly from my perspective: I need time.
Нужно время… – more impersonal and general: Time is needed (in order) to understand this rule / It takes time to understand this rule.

So:

Мне нужно время, чтобы понять это правило.
– Focus on you personally.
Нужно время, чтобы понять это правило.
– General statement: anyone would need time for that.