Если тебе что‑то непонятно, подойди к учителю после урока.

Because Russian says кому-то понятно / непонятно — literally, to someone it is clear / unclear.

So:

• тебе = to you (dative case)
• not ты = you (nominative)

This is a very common pattern in Russian:

• Мне понятно. = It’s clear to me.
• Ему непонятно. = It’s unclear to him.
• Тебе всё ясно? = Is everything clear to you?

So тебе что-то непонятно is the normal Russian way to say something is unclear to you / you don’t understand something.

In the present tense, Russian usually leaves out the verb to be.

So where English says:

• something is unclear
• you are tired
• he is a teacher

Russian often says simply:

• что-то непонятно
• ты устал
• он учитель

The idea of is/are/am is understood from context.

If the sentence were in the past or future, Russian would show it:

• Если тебе что-то было непонятно... = If something was unclear to you...
• Если тебе что-то будет непонятно... = If something is unclear / will be unclear to you...

Что-то means something.

It is made from:

• что = what
• -то = a particle that makes it indefinite

So:

• что = what
• что-то = something

The hyphen is part of the standard spelling of this type of word in Russian:

• кто-то = someone
• где-то = somewhere
• как-то = somehow
• что-то = something

In this sentence, что-то непонятно means something is unclear.

Because here the word means is unclear, not unclear + noun.

Compare:

• непонятный вопрос = an unclear question
  
  • непонятный is a full adjective modifying вопрос
• вопрос непонятен / это непонятно = the question is unclear / this is unclear
  
  • this is the predicative use: the word is part of the predicate

In your sentence, непонятно is the natural form for is unclear.

A useful way to remember it is:

• непонятный = unclear (before a noun)
• непонятно = unclear / not clear (as the main idea of the sentence)

So:

• непонятное слово = an unclear word
• мне это непонятно = this is unclear to me

Because Russian often uses the neuter form in impersonal or general statements like this, especially with words meaning clear, necessary, possible, interesting, and so on.

Examples:

• Мне интересно. = It’s interesting to me.
• Нам важно. = It’s important to us.
• Тебе понятно? = Is it clear to you?
• Тебе что-то непонятно? = Is something unclear to you?

Also, что-то is grammatically neuter, so непонятно fits naturally here.

For a learner, the simplest takeaway is:
мне / тебе / ему + понятно / непонятно is a standard Russian pattern.

Yes, если means if.

In this sentence it introduces a condition:

• Если тебе что-то непонятно... = If something is unclear to you...

In English, instructions like this sometimes sound close to when in meaning:

• If you don’t understand something, come to the teacher after class.

That means whenever this happens, not necessarily a doubtful hypothetical situation.

Russian still uses если here.

Because the sentence has two parts:

1. Если тебе что-то непонятно = subordinate clause
2. подойди к учителю после урока = main clause

Russian normally separates an если clause from the main clause with a comma.

So:

• Если тебе что-то непонятно, подойди...

This is very similar to English punctuation in sentences like:

• If you have questions, ask me later.

Подойди comes from подойти, which means to come up to, approach, or walk over to someone.

So:

• иди = go / walk
• подойди = come up to, approach

In this context, подойди к учителю means:

• go over to the teacher
• come up to the teacher

That is more natural than plain иди, because the idea is specifically approach the teacher.

Also, подойти is perfective, which is very common in instructions when you mean one complete action:

• подойди = go up once / do that action

This is an aspect question.

• подойди = perfective imperative
• подходи = imperfective imperative

In commands, the perfective often means do this one complete action:

• Подойди к учителю. = Go up to the teacher.

The imperfective can sound more like:

• a repeated/habitual instruction,
• a process,
• or sometimes a softer invitation depending on context.

Here the speaker means a single concrete action after the lesson, so подойди is the natural choice.

It is dative singular.

The verb подойти normally goes with к + dative:

• подойти к кому? к чему? = to approach whom? what?

So:

• к учителю = to the teacher
• к маме = to mom
• к окну = to the window

The noun changes like this:

• учитель = teacher
• к учителю = to the teacher

So подойди к учителю literally means approach the teacher.

Because после requires the genitive case.

So:

• после чего? = after what?

And:

• урок = lesson
• урока = of the lesson / after the lesson (genitive singular)

Other examples:

• после обеда = after lunch
• после школы = after school
• после работы = after work

So после урока is the correct form.

It can often be translated as either lesson or class, depending on context.

• In a school context, после урока often means after class.
• More literally, урок is lesson.

So both of these can work in English:

• after the lesson
• after class

The Russian word itself is completely normal here.

Yes. This sentence addresses one person informally.

You can see that from:

• тебе = to you (informal singular)
• подойди = imperative singular

This is what you would say to:

• one student,
• a child,
• a friend,
• someone you address with ты.

If you wanted the polite singular or plural version, you would say:

• Если вам что-то непонятно, подойдите к учителю после урока.

That means either:

• If something is unclear to you, come to the teacher after class (formal singular),
• or the same thing addressed to several people.

Yes. Russian word order is flexible, although some versions sound more natural than others depending on emphasis.

The given sentence:

• Если тебе что-то непонятно, подойди к учителю после урока.

is very natural.

You could also hear:

• Если что-то тебе непонятно, подойди к учителю после урока.

That puts slightly more focus on что-то.

In general:

• тебе что-то непонятно = a neutral, common word order
• что-то тебе непонятно = also correct, with a slightly different emphasis

So the word order here is not random, but it is not as rigid as English word order either.