Если на стекле появится трещина, его лучше заменить.

For a visible crack on a pane or surface, Russian normally says на стекле.

The idea is that the crack is thought of as something that appears on the glass surface. English also often says a crack in the glass, so this can feel different to learners.

• на стекле = the normal way to say there is a crack on the glass / in the pane
• в стекле can exist in other contexts, but it is not the most natural choice here

So in this sentence, на стекле появится трещина is the idiomatic wording.

Because трещина is the subject of появится.

In на стекле появится трещина:

• трещина = the thing that appears
• появится = will appear
• на стекле = a location phrase, not the subject

So Russian uses the nominative because трещина is doing the action grammatically.

Появится is the future tense of the perfective verb появиться.

That makes sense here because the sentence is talking about a single completed event:

• first, a crack appears
• then, replacing the glass is recommended

Perfective is natural because the crack is treated as something that comes into existence at a specific moment.

Compare:

• появится = will appear / appears as a one-time event
• появляется = appears in a general, repeated, or ongoing sense

So Если на стекле появится трещина... means If a crack appears on the glass..., referring to a real future possibility.

This is a very common question. Russian and English behave differently here.

In English, real future conditions usually use the present after if:

• If a crack appears, ...

But Russian normally uses future when the meaning is future:

• Если на стекле появится трещина, ...

So this is completely normal Russian. It does not sound strange or overly literal.

A good rule is:

• English: If + present, then future in the main clause
• Russian: Если + future, then usually future or an infinitive-based recommendation in the main clause

Оно is the nominative form, but here the pronoun is the direct object of заменить, so Russian uses the accusative form его.

For стекло:

• nominative: оно
• accusative: его

So:

• стекло = the noun
• его = it in object position

That is why его лучше заменить means it is better to replace it.

Yes, the sentence is impersonal.

Его лучше заменить literally means something like:

• It is better to replace it
• It’s better to replace it
• One should better replace it

Russian often makes recommendations without saying who exactly should do the action. That is very natural.

So there is no need for a subject like вы, мы, or кто-то. The sentence simply states the best course of action.

Because the sentence recommends a complete action: replacing the glass.

• заменить = to replace, as a completed act
• менять = to replace/change in a more general, repeated, or process-oriented sense

Here the meaning is not it is better to be replacing it or it is better to replace it regularly. It means that once there is a crack, the right thing is to replace the glass.

So лучше заменить is the natural choice.

Because Если на стекле появится трещина is a subordinate clause introduced by если.

Russian separates that clause from the main clause with a comma:

• Если на стекле появится трещина, его лучше заменить.

This is standard punctuation for conditional clauses.

Стекло can mean:

• glass as a material
• a pane of glass
• a window pane
• a car window or windshield, depending on context

In this sentence, it usually means a specific piece of glass that can be replaced, such as:

• a window pane
• a screen cover
• a car window / windshield
• some other glass panel

So the exact English word depends on context, even though the Russian word stays стекло.

Yes, absolutely. That is a very natural alternative.

The difference is:

• Если на стекле появится трещина = If a crack appears on the glass
• Если стекло треснет = If the glass cracks

The second version is a bit more direct and compact. The original sentence focuses on the appearance of a crack; the alternative focuses on the glass cracking itself.

Both are correct and natural.