Se o copo cair no chão, eu vou pegar outro.

Because Portuguese often uses the future subjunctive after se when talking about a possible future event.

So:

• Se o copo cair... = If the glass falls / if the glass should fall
• cair here is the future subjunctive form of cair

This is very common in Portuguese:

• Se ele chegar cedo, saímos. = If he arrives early, we leave.
• Se chover, fico em casa. = If it rains, I’ll stay home.

For English speakers, this can feel strange because English does not have a special verb form here. Portuguese does.

A quick comparison:

• Se o copo cair no chão, eu vou pegar outro.
  = a future possibility
• Se o copo cai no chão...
  sounds nonstandard in many contexts or can suggest a more habitual/colloquial idea in some varieties, not the neutral standard choice here.

So in standard Brazilian Portuguese, se + future possibility usually calls for the future subjunctive.

It is the future subjunctive of cair.

For many verbs, the future subjunctive looks different from the infinitive, but with cair, the form happens to be the same:

• infinitive: cair
• future subjunctive: cair

That is why it may not look like a changed form, even though grammatically it is.

A few examples with other verbs make this clearer:

• fazer → se eu fizer = if I do
• ter → se eu tiver = if I have
• ir → se eu for = if I go

So in your sentence, cair is not just the plain dictionary form by accident; it is functioning as the future subjunctive.

Both are possible, but eu vou pegar is much more common in everyday Brazilian Portuguese.

• eu vou pegar outro = I’m going to get another one
• pegarei outro = I will get another one

The simple future (pegarei) is grammatical, but it often sounds:

• more formal
• more written
• less conversational

In normal spoken Brazilian Portuguese, the ir + infinitive structure is extremely common for future meaning.

So this sentence sounds natural and everyday:

• Se o copo cair no chão, eu vou pegar outro.

A more formal version could be:

• Se o copo cair no chão, pegarei outro.

Yes. In Brazilian Portuguese, subject pronouns are often omitted when the verb already makes the subject clear.

So both of these are natural:

• Se o copo cair no chão, eu vou pegar outro.
• Se o copo cair no chão, vou pegar outro.

The second version is probably more common in casual speech.

Including eu can add:

• emphasis
• contrast
• clarity

For example:

• Se o copo cair no chão, eu vou pegar outro, mas você limpa.
  = If the glass falls on the floor, I’ll get another one, but you clean up.

So eu is optional here, not wrong.

Because no is the contraction of:

• em + o = no

So:

• no chão = on the floor / onto the floor depending on context

Portuguese very often contracts em with definite articles:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

Examples:

• no carro = in the car
• na mesa = on the table
• nos livros = in the books
• nas caixas = in the boxes

So cair no chão is the normal form.

Yes. Chão is a masculine noun, so it takes o:

• o chão = the floor / the ground

That is why the contraction is no chão:

• em + o chão → no chão

If the noun were feminine, you would use na instead:

• na mesa = on the table

In this sentence, it is best understood as onto the floor or simply on the floor after falling.

Portuguese often uses cair em / no with places where something ends up after falling:

• cair no chão = fall on the floor / fall to the floor
• cair na água = fall into the water

English distinguishes very sharply between location and movement in some cases, but Portuguese often uses the same preposition naturally in these contexts.

So Se o copo cair no chão means the glass ends up on the floor.

Because Portuguese can omit the noun when it is already understood from context.

Here:

• outro = another one
• specifically, another glass

So:

• eu vou pegar outro = I’ll get another one
• eu vou pegar outro copo = I’ll get another glass

Both are correct. The version without copo sounds natural because the noun has already been mentioned.

This works a lot like English:

• I need another.
• I need another glass.

Here, pegar means something like:

• get
• grab
• pick up
• sometimes take

In this sentence, vou pegar outro most naturally means:

• I’ll get another one

It does not necessarily mean physically picking the fallen glass up off the floor. It means getting a replacement glass.

That is important, because pegar has a wide range of meanings in Portuguese depending on context.

Examples:

• Vou pegar um copo. = I’m going to get a glass.
• Peguei o ônibus. = I caught the bus.
• Peguei meu celular. = I picked up / took my phone.

So here, think of pegar outro as get another one.

Yes, but it changes the nuance a little.

• vou pegar outro = I’ll get another one
• vou buscar outro = I’ll go get another one / fetch another one

Buscar often emphasizes going to retrieve something from somewhere.
Pegar is broader and very common in everyday speech.

In this sentence, pegar outro sounds very natural and idiomatic.

The comma separates the if-clause from the main clause:

• Se o copo cair no chão, = if the glass falls on the floor
• eu vou pegar outro. = I’ll get another one

This is similar to English, where we usually write:

• If the glass falls on the floor, I’ll get another one.

So the comma helps mark the introductory conditional clause.

It is a normal, open possibility in the future.

The sentence does not mean the speaker thinks the glass definitely will fall, and it does not sound extremely imaginary either. It simply means:

• if that happens, then this is what I’ll do

This is one of the most common uses of se + future subjunctive in Portuguese: a possible future condition.

Yes, but caso sounds a bit more formal or careful.

• Se o copo cair no chão, eu vou pegar outro.
  = normal, everyday
• Caso o copo caia no chão, eu vou pegar outro.
  = in case the glass falls on the floor

Notice that with caso, the verb usually goes in the present subjunctive:

• caso ... caia

So both are correct, but se is the more common everyday choice here.

Because the sentence is talking about a specific glass, not just any glass in general.

• o copo = the glass

In Portuguese, definite articles are used very naturally in many places where English may or may not use them. Here, though, English would also normally use the:

• If the glass falls on the floor...

So o copo is exactly what you would expect for a specific glass in the situation.

A more formal version could be:

• Se o copo cair no chão, pegarei outro.

This uses the simple future pegarei instead of vou pegar.

Another slightly fuller version could be:

• Se o copo cair no chão, eu pegarei outro copo.

But in normal spoken Brazilian Portuguese, the original sentence is more natural:

• Se o copo cair no chão, eu vou pegar outro.

Cair is usually pronounced roughly like kah-EER, with two syllables:

• ca-ir

Important points:

• the a sounds like the a in father
• the stress falls on the second syllable: -ir
• the final r in Brazilian Portuguese is often softer than an English r, and in many accents it sounds more like an h sound at the end of the word

So depending on accent, it may sound approximately like:

• ka-EE(h)

You do not pronounce it like one single English syllable such as care. It clearly has two syllables.

Yes, that is the natural implication.

If someone says:

• Se o copo cair no chão, eu vou pegar outro.

the idea is usually:

• the glass may break
• or it may get dirty
• or it will no longer be suitable to use

So the speaker plans to replace it with another glass.

Even though the sentence does not explicitly say break, that is a very natural implied meaning from the context.