Si no se fía usted de ese enlace, no lo abra y pregunte en la oficina.

Because the verb here is fiarse de, which means to trust or to rely on.

This is a pronominal verb, so it normally includes a pronoun:

• me fío
• te fías
• se fía
• nos fiamos

So si no se fía usted de ese enlace means if you do not trust that link.

Without se, fiar usually has other meanings, such as to sell on credit or to entrust something.

Because fiarse is normally followed by de when you say what or whom you trust.

So:

• fiarse de alguien = to trust someone
• fiarse de algo = to trust something

Examples:

• No me fío de él. = I don’t trust him.
• ¿Te fías de esta página web? = Do you trust this website?

So de ese enlace is required by the verb.

Usted is the formal singular you.

The sentence is giving advice or an instruction politely/formally:

• no lo abra
• pregunte

Those are usted command forms, so the sentence is addressing one person formally.

In English, we just say you, but Spanish distinguishes:

• tú = informal singular
• usted = formal singular

Spanish word order is more flexible than English.

You could also say:

• Si usted no se fía de ese enlace...

That is just as correct.

Putting usted after the verb can sound a bit more natural or less heavy in some contexts. Spanish often moves subject pronouns around because the verb already gives a lot of information.

So both are possible:

• si usted no se fía...
• si no se fía usted...

Yes, very often.

Spanish often leaves out subject pronouns when they are not needed. You could say:

• Si no se fía de ese enlace, no lo abra y pregunte en la oficina.

That still works.

However, se fía could in isolation mean he/she/you (formal) trusts, so adding usted makes the intended subject extra clear. It can also reinforce the formal tone.

Spanish has a three-way contrast with demonstratives:

• este = this, near the speaker
• ese = that, near the listener or already mentioned
• aquel = that over there, farther away from both

So ese enlace means that link.

In a sentence like this, ese often refers to a link the other person is looking at or has received. It is the natural choice for that link.

Lo is a direct object pronoun meaning it.

It replaces ese enlace:

• No abra ese enlace.
• No lo abra.

Since enlace is masculine singular, the pronoun is lo.

Compare:

• el enlace → lo
• la página → la

So no lo abra means don’t open it.

Because with a negative command, object pronouns go before the verb.

So:

• Ábralo. = Open it.
• No lo abra. = Don’t open it.

This is a very common pattern in Spanish:

• Hágalo. / No lo haga.
• Léalo. / No lo lea.

Because these are usted commands, and usted commands use the present subjunctive form.

For the verb abrir:

• tú command: abre
• usted command: abra

For the verb preguntar:

• tú command: pregunta
• usted command: pregunte

In this sentence:

• no lo abra = formal singular negative command
• pregunte en la oficina = formal singular affirmative command

So the forms show that the speaker is addressing one person formally.

Here en la oficina means at the office or in the office.

The idea is ask there / ask at that place.

Spanish often uses preguntar en + place when you mean ask at a location:

• Pregunte en recepción. = Ask at reception.
• Pregunté en la tienda. = I asked at the shop.

If you say whom you ask, Spanish often uses a:

• Pregunte a la secretaria. = Ask the secretary.

So:

• preguntar en la oficina = ask at the office
• preguntar a alguien = ask someone

Because the sentence begins with a conditional clause:

• Si no se fía usted de ese enlace, ...

Spanish often uses a comma after an introductory si clause, especially when it comes before the main clause.

So the structure is:

• If you don’t trust that link, don’t open it and ask at the office.

The comma helps separate:

1. the condition
2. the main instruction

It is standard and natural here.