Antes da corrida, eu bebo um copo de água.

Because de + a normally contracts to da in Portuguese.

• antes de = before
• a corrida = the run / the race
• antes da corrida = before the run

This kind of contraction is very common:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

So antes da corrida is the natural standard form.

It can mean either, depending on context.

In Antes da corrida, eu bebo um copo de água, corrida could mean:

• a run (as exercise)
• a race (a sporting event)

If the meaning has already been given to the learner, context will decide which one is intended. Portuguese often uses corrida in both senses.

Yes, it can often be omitted.

Portuguese verbs usually show the subject clearly, so bebo already means I drink. That means both of these are possible:

• Antes da corrida, eu bebo um copo de água.
• Antes da corrida, bebo um copo de água.

Both are correct. Including eu can add:

• emphasis
• contrast
• clarity

In many everyday situations, Portuguese speakers would simply say Antes da corrida, bebo um copo de água.

Here bebo is the present indicative of beber and normally means I drink.

In this sentence, it usually expresses a habit or routine:

• Before the run, I drink a glass of water.

European Portuguese often uses the simple present for habitual actions, just like English does.

If you wanted to say I am drinking right now, Portuguese would more naturally use a different structure, such as:

• Estou a beber = I am drinking

So in this sentence, bebo is best understood as a habitual present.

Because um copo de água means a glass of water, which is a specific quantity.

Compare:

• Bebo água. = I drink water.
• Bebo um copo de água. = I drink a glass of water.

So um copo de água is more precise.

Also, Portuguese often uses de in this kind of expression:

• um copo de água = a glass of water
• uma chávena de café = a cup of coffee
• uma garrafa de vinho = a bottle of wine

Because in Portuguese, after a measure/container expression like um copo de, the noun often appears without an article.

So:

• um copo de água = a glass of water
  not usually
• um copo da água

The phrase works like a unit: container + de + substance.

You see the same pattern in:

• um copo de leite = a glass of milk
• uma garrafa de água = a bottle of water
• um prato de sopa = a plate/bowl of soup

Because Antes da corrida is an introductory time phrase.

The comma helps separate the time setting from the main clause:

• Antes da corrida, eu bebo um copo de água.

This is similar to English:

• Before the run, I drink a glass of water.

The comma is very natural here, especially in writing. In shorter sentences, punctuation can sometimes vary, but this version is perfectly standard.

Yes, and the meaning changes slightly.

• Antes da corrida = before the run / race
• Antes de correr = before running

The first uses a noun (corrida).
The second uses an infinitive verb (correr).

Both are natural, but they focus on slightly different things:

• Antes da corrida points to the event itself.
• Antes de correr points to the action of running.

In modern standard Portuguese, you normally write de água.

You may sometimes see forms like d’água in:

• poetry
• song lyrics
• literary style
• some fixed expressions or names

But for normal standard writing, especially for learners, use:

• um copo de água

That is the safest and most standard choice.

The word is água, and the accent shows that the stress falls on the first syllable:

• Á-gua

The á is stressed. The gua is unstressed.

A rough guide for English speakers is something like AH-gwah, though not exactly.

In European Portuguese, the pronunciation is more reduced and compact than many English speakers expect, but the key point for learners is:

• stress the first syllable
• do not stress the end

Yes, it is completely natural.

Antes da corrida, eu bebo um copo de água. is correct and idiomatic in European Portuguese.

A very common slightly less explicit version would be:

• Antes da corrida, bebo um copo de água.

Both are fine. The original sentence sounds natural and is a good model for learners.

Yes. Portuguese allows some flexibility.

For example:

• Antes da corrida, eu bebo um copo de água.
• Eu bebo um copo de água antes da corrida.
• Bebo um copo de água antes da corrida.

All of these are correct.

The difference is mostly about focus and style:

• putting Antes da corrida first emphasizes the time
• putting it later makes the sentence feel a bit more neutral in flow

For a learner, all of these are useful and natural.