Prefiero pagar en efectivo cuando compro fruta en el mercado.

Because prefiero is the 1st-person singular present-tense conjugation of preferir (yo prefiero = “I prefer”). After yo, Spanish typically drops the subject pronoun, so Prefiero alone already means “I prefer.”

Yes. Preferir is a stem-changing verb (e → ie) in most present-tense forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos (no stem change)
• ellos prefieren

So the e in the stem changes to ie in prefiero.

After verbs of preference like preferir, Spanish commonly uses infinitive when the subject stays the same:

• Prefiero pagar... (I prefer to pay...) If the subject changes, you often use que + subjunctive:
• Prefiero que pagues en efectivo. (I prefer that you pay in cash.)

En efectivo is a fixed expression meaning “in cash / with cash.” Spanish commonly uses en + noun to express “by means of / in the form of” for payments:

• pagar en efectivo (pay in cash)
• pagar en tarjeta / con tarjeta (pay by card / with a card)

In Latin America, en efectivo is very standard.

You’ll most often hear en efectivo.
Con efectivo is possible but less idiomatic. If you want to be extra clear, you can say:

• pagar en efectivo
• pagar con dinero en efectivo (more explicit, a bit heavier)

Here cuando + present indicative describes a habitual/regular situation (“whenever/when I buy…”), so compro (present indicative) is correct.

Spanish tends to use:

• cuando + present indicative for habits and known routines: cuando compro...
• cuando + subjunctive for future, unknown, or not-yet-real situations: cuando compre... (when I buy [in the future])

Example contrast:

• Habit: Cuando compro fruta, pago en efectivo.
• Future: Cuando compre fruta mañana, pagaré en efectivo.

Yes, but it changes the meaning/feel:

• cuando compro = “when/whenever I buy” (habitual, general)
• cuando compre = “when I buy (at some future time)” (not yet happened; more future-oriented)

Your sentence with cuando compro sounds like a general preference in repeated situations.

In Spanish, a singular noun can be used as a general/uncountable category:

• compro fruta = “I buy fruit” (fruit in general)

Adding an article can shift the meaning:

• compro la fruta often implies specific fruit (e.g., the fruit we discussed, or the fruit at a particular stall)
• compro frutas can emphasize variety or multiple pieces/types, but fruta is perfectly natural as a category.

Because en el mercado describes location: you buy fruit in the market.

al mercado (a + el) means movement/destination: going to the market.

• Compro fruta en el mercado. (I buy fruit in the market.)
• Voy al mercado. (I go to the market.)

You can combine them:

• Voy al mercado y compro fruta allí.

El is the masculine singular definite article (“the”). With many specific places in context (like “the market” you go to), Spanish commonly uses the article:

• en el mercado
• en el supermercado

Sometimes you can omit the article in certain set phrases or more general references, but en el mercado is the normal choice.

Spanish verb endings usually show the subject, so prefiero already signals “I.” You add yo mainly for emphasis or contrast:

• Neutral: Prefiero pagar en efectivo...
• Emphasis/contrast: Yo prefiero pagar en efectivo, pero ella prefiere pagar con tarjeta.

Yes. Spanish allows that kind of reordering:

• Prefiero pagar en efectivo cuando compro fruta en el mercado.
• Cuando compro fruta en el mercado, prefiero pagar en efectivo.

Both are correct; starting with the cuando clause can sound a bit more structured or explanatory.