Si la plancha está demasiado caliente, la costura puede estropearse.

Because caliente describes a temporary condition here: the iron’s temperature at that moment.

• estar caliente = to be hot
• ser is usually used for more permanent or defining characteristics

So in this sentence, la plancha está demasiado caliente means the iron is too hot right now, not that being hot is part of its essential nature.

Demasiado means too, while muy means very.

• muy caliente = very hot
• demasiado caliente = too hot

The sentence is warning about a negative consequence, so demasiado is the correct choice: the iron is hotter than it should be.

Spanish often uses the definite article (el, la, los, las) more than English does, especially when talking about things in a general way.

So:

• la plancha = the iron
• la costura = the seam / stitching

In English, you might say If an iron is too hot, the seam can get damaged or just If the iron is too hot..., but Spanish naturally keeps the articles here.

Here, la plancha means the iron used for clothes.

Be careful, because plancha can have other meanings depending on context, such as:

• a metal plate
• a griddle / hot plate
• a bodybuilding exercise (plank), in some contexts

But in a sentence about heat and sewing, la plancha clearly means a clothes iron.

Costura can mean several related things:

• sewing
• stitching
• seam

In this sentence, it most naturally refers to the seam or the stitching in the fabric.

So la costura puede estropearse means the seam/stitching can be damaged or ruined.

It is made of:

• puede = can / may
• estropearse = to get damaged / to be ruined / to spoil

So literally, it is can spoil itself, but in natural English that becomes:

• can get damaged
• can be ruined

The -se in estropearse shows that the damage happens to the seam itself. This is very common in Spanish with verbs that describe things changing state:

• romperse = to break
• quemarse = to get burned
• estropearse = to get damaged / go bad

Because estropear usually means to damage something:

• La plancha puede estropear la costura.
  = The iron can damage the seam.

But estropearse means to get damaged:

• La costura puede estropearse.
  = The seam can get damaged.

So the version in your sentence focuses on what happens to the seam, not directly on the iron as the subject causing the damage.

Yes. That is also correct, but the focus changes slightly.

• La costura puede estropearse
  = The seam can get damaged.
  Focus: the seam and its condition.

• Puede estropear la costura
  = It can damage the seam.
  Focus: the iron as the thing causing the damage.

Both are natural. The original sentence sounds a little less direct and more descriptive.

Because this is a real or possible condition, not a hypothetical one.

Spanish uses the present indicative after si for real possibilities:

• Si llueve, no salimos.
  = If it rains, we’re not going out.
• Si la plancha está demasiado caliente, la costura puede estropearse.
  = If the iron is too hot, the seam can get damaged.

You do not use the subjunctive after si in this kind of sentence.

Yes. You can reverse the clauses:

• Si la plancha está demasiado caliente, la costura puede estropearse.
• La costura puede estropearse si la plancha está demasiado caliente.

Both mean the same thing. The first version emphasizes the condition first; the second emphasizes the possible result first.

Yes, very common. In Spain, estropearse is a very natural everyday verb for things becoming damaged, ruined, or broken.

Examples:

• El móvil se ha estropeado.
  = The phone has broken / stopped working.
• La comida se ha estropeado.
  = The food has gone bad.

In some other Spanish-speaking regions, people may also use verbs like dañarse, arruinarse, or descomponerse, depending on the situation. But estropearse is perfectly standard and especially common in Spain.

In natural speech, the words link together, so it may sound more fluid than it looks.

A rough guide:

• puede ≈ PWEH-de
• estropearse ≈ es-tro-pe-AR-se

Together:

• PWEH-de es-tro-pe-AR-se

In Spain, the s is clearly pronounced, and the r in estropearse is a light tap. The stress falls on:

• puEde
• estropeArse

So the strongest syllables are pue- and -ar-.