Siempre miro la fecha de caducidad antes de comprar leche en el supermercado.

In this sentence, mirar means to look at or to check, which fits the idea of deliberately examining the expiry date.

• Mirar = to look at, to watch, to check
• Ver = to see

So Siempre miro la fecha de caducidad... means the speaker actively checks it, not just happens to see it.

La fecha de caducidad means the expiry date or use-by date.

It is a very common expression on food packaging in Spain.

Breakdown:

• fecha = date
• de = of
• caducidad = expiration, expiry

So literally it is the date of expiry.

In some other Spanish-speaking countries, you may also hear fecha de vencimiento.

Spanish usually uses the definite article more often than English.

Here, la fecha de caducidad refers to the expiry date on the product, so Spanish naturally says the expiry date, even where English might simply say I always check expiry dates or I always check the expiry date depending on context.

So:

• miro la fecha de caducidad = I check the expiry date

Using la is normal and expected here.

After antes de, Spanish uses either:

• a noun
• or an infinitive

Since comprar is an infinitive, antes de comprar means before buying or before I buy.

Examples:

• antes de salir = before leaving
• antes de comer = before eating
• antes de comprar leche = before buying milk

So antes de + infinitive is a very common structure.

Spanish often leaves out subject pronouns because the verb ending already shows who is doing the action.

• miro = I look / I check

Because -o tells you it is yo, adding yo is usually unnecessary unless you want emphasis or contrast.

So:

• Siempre miro... = normal
• Yo siempre miro... = more emphatic, like I always check...

Here leche is being used in a general, non-specific way, so Spanish often leaves out the article.

• comprar leche = to buy milk
• comprar pan = to buy bread
• comprar fruta = to buy fruit

If you said comprar la leche, it would usually mean to buy the milk, referring to a specific milk already known in the context.

So in this sentence:

• comprar leche = buying milk in general

Yes. Siempre is flexible, although the version in your sentence is very natural.

Common possibilities:

• Siempre miro la fecha de caducidad...
• Miro siempre la fecha de caducidad...
• Yo siempre miro la fecha de caducidad...

The first one is the most neutral and common.

Putting siempre at the beginning gives it a little prominence:

• Siempre miro... = I always check...

Because en indicates location: in / at the supermarket.

• en el supermercado = in the supermarket
• al supermercado = to the supermarket

So:

• compro leche en el supermercado = I buy milk in the supermarket
• voy al supermercado = I go to the supermarket

In your sentence, the speaker is talking about where the buying happens, not movement toward the supermarket.

No. In this sentence, en el supermercado most naturally goes with comprar leche, not with miro.

So the meaning is:

• I always check the expiry date before buying milk at the supermarket

In other words, the checking happens as part of the shopping process in the supermarket.

The present tense in Spanish is often used for habits and routines.

So Siempre miro... means:

• I always check...
• a habitual action, not just something happening right now

This is exactly like English using the present simple for habits:

• I always check the expiry date

You could, but it changes the nuance.

• miro la fecha de caducidad = I look at / check the expiry date
• veo la fecha de caducidad = I see the expiry date

If you want to express a habit of intentionally checking it, miro is much better.

Because de has different functions in each expression.

1. fecha de caducidad

   • here de means something like of
   • fecha de caducidad = date of expiry

2. antes de comprar

   • here de is part of the fixed structure antes de + infinitive
   • antes de comprar = before buying

So both use de, but for different grammatical reasons.

Yes, it sounds very natural in Spain.

A few notes:

• supermercado is universal
• fecha de caducidad is very common in Spain
• in some Latin American countries, fecha de vencimiento may be more common

So the sentence is perfectly good Spanish, and it sounds especially natural for Spain.

Yes, but it means something slightly different.

• comprar leche = buy milk in general
• comprar la leche = buy the milk, usually a specific milk already understood in context

For example:

• Siempre miro la fecha de caducidad antes de comprar leche = general habit
• Siempre miro la fecha de caducidad antes de comprar la leche = before buying the milk, perhaps the milk I came to get

Your original sentence is more natural for a general statement.

No, but it is very commonly used with food, medicine, documents, and anything that can expire.

Examples:

• fecha de caducidad de la leche = expiry date of the milk
• fecha de caducidad del yogur = expiry date of the yogurt
• caducidad del pasaporte = passport expiry

In this sentence, it clearly refers to food expiry.

Yes, and in this sentence I check is probably the most natural English translation.

Literally:

• miro = I look at

But in context:

• Siempre miro la fecha de caducidad... = I always check the expiry date...

That is a good example of how the most natural translation is not always the most literal one.