Learn Spanish (Latin America): Ellos prefirieron ese jabón.

Preferir (to prefer) is an -ir verb. In the preterite (past) tense, -ir verbs that have a stem change in the present also get a stem change in the third-person forms like "they" (ellos). Instead of keeping the "e", it changes to "i", making it prefirieron.

How do I know which "e" in preferir changes to an "i"?

When a verb has multiple vowels in the stem, the change always happens to the vowel closest to the ending. In preferir, it is the second "e" that changes, giving us the root prefir- before adding the -ieron ending.

I often mix up ese and eso. Why do we use ese here?

Ese means "that" when pointing out a specific masculine singular noun, like ese jabón (that soap). You only use eso when talking about an abstract idea, a general situation, or an unidentified object. Eso never goes directly in front of a noun.

Why does jabón have an accent mark on the "o"?

In Spanish, words ending in an "n", "s", or a vowel normally have the spoken stress on the second-to-last syllable. Since the natural stress in jabón falls on the last syllable instead, it needs a written accent mark to break the normal rule and tell you where to emphasize the word.