Если в предложении нет запятой, его смысл иногда меняется.

Если means if and introduces a condition. In this sentence, the first part sets up the condition:

Если в предложении нет запятой = If there is no comma in the sentence

This is a very common Russian pattern:

Если + condition, main clause

For example:

Если идёт дождь, мы остаёмся дома.
= If it is raining, we stay home.

So here, Если tells you that the second part depends on the first.

Because after в meaning in, Russian usually uses the prepositional case when talking about location.

• dictionary form: предложение
• after в for location: в предложении

So:

• в предложении = in the sentence
• в книге = in the book
• в городе = in the city

If в showed motion into something, then a different case could appear, but here it is static location: the comma is missing in the sentence.

Russian often uses нет to express there is no / there isn't.

So:

• нет запятой = there is no comma
• literally, this is the standard Russian way to say that something is absent

This is much more natural than trying to translate English there is not word for word.

A few similar examples:

• В комнате нет стола. = There is no table in the room.
• У меня нет времени. = I don't have time.

So in your sentence, нет is the normal way to say that the comma is missing.

Because нет requires the genitive case.

The dictionary form is:

• запятая = comma

But after нет, it changes to genitive singular:

• нет запятой = there is no comma

This is a very important pattern in Russian:

• нет времени
• нет книги
• нет человека
• нет запятой

So the ending changes because the noun is no longer in the basic nominative form.

Because Russian normally puts a comma between a subordinate clause and the main clause.

Here the subordinate clause is:

Если в предложении нет запятой

And the main clause is:

его смысл иногда меняется

Russian punctuation is stricter than English in many cases. With если, a comma is normally required between the two parts.

So:

Если ..., ...

This is standard Russian punctuation.

Interestingly, the sentence itself is about how punctuation can change meaning, so the comma in the sentence is doing exactly what Russian grammar requires.

Его means its here, and it refers to предложение.

So:

• предложение = sentence
• его смысл = its meaning, meaning the meaning of the sentence

Even though его often means his, it can also mean its when it refers to a masculine or neuter noun. Since предложение is neuter, его works here.

You can think of it as:

его смысл = the sentence's meaning

Because предложение is a neuter noun.

Russian pronouns reflect the gender of the noun they refer to:

• masculine: его
• neuter: его
• feminine: её

Since предложение is neuter, the correct form is его.

Compare:

• книга → её обложка = its cover / her cover depending on context
• стол → его ножка = its leg
• предложение → его смысл = its meaning

So её would be wrong because предложение is not feminine.

Because смысл is the subject of the verb меняется.

The part

его смысл иногда меняется

literally works like:

its meaning sometimes changes

Here, meaning is the thing doing the changing, so it is the grammatical subject, and subjects are normally in the nominative case.

That is why you see:

• смысл not смысла or another case form

The verb меняется comes from меняться, which means to change in the sense of to change itself / to become different.

This is different from менять, which usually means to change something.

Compare:

• Я меняю план. = I change the plan.
  Here, someone changes an object.

• План меняется. = The plan changes.
  Here, the plan itself changes.

So in your sentence:

его смысл иногда меняется

means that the meaning itself changes. That is why the reflexive-style form меняться is used.

In Russian, the present tense is often used for general truths, habits, and things that happen sometimes or usually.

So:

его смысл иногда меняется

does not mean only it is changing right now. It means:

its meaning sometimes changes

This is the normal tense choice for a general fact.

English does the same thing:

• Water boils at 100°C.
• People make mistakes.
• Meaning sometimes changes.

So the present tense here is completely natural.

Because Russian word order is flexible, and adverbs like иногда often go before the verb.

So:

его смысл иногда меняется

is a natural way to say:

its meaning sometimes changes

You could move иногда in some contexts, but the original order sounds neutral and standard.

For example, these are possible, though they may have slightly different emphasis:

• Его смысл иногда меняется.
• Иногда его смысл меняется.

The version in the sentence is straightforward and unmarked.

Yes. Russian could also use:

Если в предложении нет запятой, смысл предложения иногда меняется.

That would mean almost the same thing.

The version with его смысл is more compact and avoids repeating предложение. Russian often does this when the reference is clear.

So both are possible, but:

• его смысл sounds smoother because it avoids repetition
• смысл предложения is slightly more explicit

Запятая specifically means comma.

It does not mean punctuation in general. For punctuation as a whole, Russian uses words like:

• пунктуация = punctuation
• знак препинания = punctuation mark

So here the sentence is specifically talking about the absence of a comma, not punctuation in general.