Lasi voi haljeta, jos laitan siihen liian kuumaa vettä.

Because voi is a modal verb, and in Finnish modal verbs are followed by the first infinitive of the main verb.

So:

• voi = can / may
• haljeta = to crack

Together, voi haljeta means can crack.

This is the same pattern as:

• Voin tulla = I can come
• Hän voi nähdä = He/she can see

So haljeta stays in the infinitive after voi.

Here voi expresses possibility.

So Lasi voi haljeta means something like:

• The glass can crack
• The glass may crack
• The glass might crack

It does not mean permission, and it is not mainly about the glass having some kind of ability. It is about what is possible under those conditions.

Finnish does not have a separate future tense like English does. The present tense is often used for future meaning when the context makes it clear.

So:

• jos laitan siihen... = if I put / if I am going to put into it...

Even though the action is in the future, Finnish normally just uses the present tense here.

Because jos clauses often use the indicative when the condition is presented as a real possibility.

• jos laitan siihen liian kuumaa vettä = if I put too hot water into it

If you used the conditional, the sentence would sound more hypothetical or would need a different structure.

So this sentence is talking about a realistic cause-and-effect situation: If I put water that is too hot into it, the glass may crack.

Siihen is the illative form of se, and here it means into it.

With laittaa (to put), Finnish often marks the destination of movement:

• laitan siihen vettä = I put water into it

So:

• se = it
• siihen = into it / into that

This is natural when talking about putting something into a container, like a glass.

Because vettä is in the partitive, and the adjective has to agree with it:

• kuumaa vettä = hot water
• liian kuumaa vettä = too hot water / water that is too hot

The partitive is used here because water is an uncountable substance, and the sentence is talking about some amount of water, not a whole clearly bounded object.

So:

• vettä = partitive of vesi
• kuumaa = partitive of kuuma, matching vettä

That is why both words end in -a/-ä here.

Because lasi is the subject of the sentence.

It is the thing that may crack, so it appears in the basic subject form:

• lasi = the glass
• Lasi voi haljeta = The glass may crack

There is no need for another case here because it is not the object of the verb.

There are two main reasons:

1. Water is a mass noun, so Finnish often uses the partitive for an unspecified amount.
2. With laittaa, if you are putting some amount of something somewhere, the thing being put is often in the partitive.

So:

• laitan vettä = I put some water
• laitan liian kuumaa vettä = I put water that is too hot

If the object were a definite whole item, another case could appear instead.

In this sentence, lasi most naturally means a glass as an object, especially a drinking glass or similar glass container.

That is because of the context:

• laitan siihen vettä = I put water into it

You normally put water into a glass/container, not into the material glass in general.

So although lasi can also refer to glass as a material in other contexts, here it most likely means a glass.

Yes. That is completely natural and means the same thing.

The two versions mainly differ in emphasis:

• Lasi voi haljeta, jos laitan siihen liian kuumaa vettä.
  Focus first on the result: The glass may crack...

• Jos laitan siihen liian kuumaa vettä, lasi voi haljeta.
  Focus first on the condition: If I put too hot water into it...

Both are correct.

Because jos laitan siihen liian kuumaa vettä is a subordinate clause, and Finnish normally separates subordinate clauses from the main clause with a comma.

So the structure is:

• main clause: Lasi voi haljeta
• subordinate clause: jos laitan siihen liian kuumaa vettä

This is standard Finnish punctuation.

Haljeta specifically means to crack or to split.

That fits very well here, because a glass exposed to overly hot water may develop a crack.

Related verbs:

• haljeta = to crack, split
• särkyä = to break, shatter
• rikoutua = to become broken, get damaged/broken

So:

• Lasi voi haljeta = The glass may crack
• Lasi voi särkyä = The glass may break/shatter

Haljeta is especially good if the idea is that the glass breaks because of temperature stress and cracks.