No quiero engañar a nadie; prefiero decir la verdad.

Because after querer in this sentence, Spanish uses an infinitive verb: querer + infinitive.

• quiero = I want
• engañar = to deceive / to trick

So quiero engañar literally means I want to deceive.

You cannot use the noun-like form engaño here, because engaño means deception, trick, or lie, not to deceive.

Because nadie is the direct object of engañar, and in Spanish, when the direct object is a person or refers to people, you usually need the personal a.

So:

• engañar a alguien = to deceive someone
• engañar a nadie = to deceive anyone / nobody

Even though nadie means nobody or anybody depending on context, it still refers to people, so the a is needed.

Yes, it is a double negative—but that is normal and correct in Spanish.

Spanish commonly uses negative concord, which means several negative words can appear together in the same sentence:

• No veo nada = I don’t see anything
• No conozco a nadie = I don’t know anyone
• No quiero engañar a nadie = I don’t want to deceive anyone

In English, using two negatives often sounds wrong, but in Spanish this structure is standard.

A useful rule:

• If the negative word comes after the verb, you usually also use no
• If the negative word comes before the verb, you usually do not use no

For example:

• No vino nadie
• Nadie vino

Both are correct.

Because preferir is a stem-changing verb.

In the present tense, the e in the stem changes to ie in most forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So prefiero is the correct yo form.

This is the same kind of change you see in verbs like:

• querer → quiero
• pensar → pienso
• entender → entiendo

Because after preferir, Spanish also normally uses the infinitive when the subject stays the same.

Here, the subject is I in both parts:

• prefiero = I prefer
• decir = to say / to tell

So:

• Prefiero decir la verdad = I prefer to tell the truth

Using digo would mean a fully conjugated second verb, which does not fit this structure.

Very common pattern:

• prefiero comer
• prefiero esperar
• prefiero decir la verdad

Because Spanish often uses the definite article where English does not.

Decir la verdad is a fixed and very common expression meaning:

• to tell the truth

Similarly, Spanish often says:

• tener razón = to be right
• decir la verdad = to tell the truth
• decir la mentira is much less common; usually you say decir mentiras or mentir

So la verdad is the natural phrase here.

They are related, but not identical.

• mentir = to lie
• engañar = to deceive / trick / mislead

You can mentir by saying something false. You can engañar someone in a broader way—not only with words, but also by hiding something, creating a false impression, cheating, or tricking them.

So in this sentence, engañar sounds broader and a bit stronger than just mentir.

The semicolon links two closely related ideas:

• No quiero engañar a nadie
• prefiero decir la verdad

It is stronger than a comma, but not as separate as a full stop.

In everyday Spanish, many people would also write:

• No quiero engañar a nadie. Prefiero decir la verdad.
• No quiero engañar a nadie, prefiero decir la verdad.

But the semicolon is a neat, correct way to show that the second idea contrasts with and explains the first.

In standard Spanish, especially careful or formal Spanish, no—you should say engañar a nadie.

That is because of the personal a with people.

So the standard form is:

• No quiero engañar a nadie

Leaving out the a may be heard in some contexts or varieties, but it is not the recommended standard form for learners.

Engañar is pronounced roughly like en-ga-NYAR.

Key points:

• ñ is like the ny sound in canyon
• The stress is on the last syllable: -ñar

So:

• engañar ≈ en-ga-NYAR

For Spain, the pronunciation of the rest is straightforward:

• No quiero engañar a nadie
  ≈ noh KYEH-roh en-ga-NYAR ah NAH-dyeh

That is only an approximation, but it helps.

Yes, in this specific sentence prefiero decir la verdad sounds very natural.

Both are possible, but they are slightly different:

• decir la verdad = focuses on what you say
• ser honesto = focuses on your general quality of being honest

So:

• No quiero engañar a nadie; prefiero decir la verdad
  sounds like: I don’t want to deceive anyone; I’d rather tell the truth

If you said prefiero ser honesto, it would sound more general and a little less direct.