Learn Spanish (Spain): El médico explicó que la terapia era muy útil. - The doctor explained that the therapy was very useful.

The verb era is in the Imperfect tense. When reporting what someone said in the past (using a past tense reporting verb like explicó), a statement that was originally in the Present tense shifts to the Imperfect. The doctor's original words were la terapia es muy útil (the therapy is very useful). Because we are reporting it after the fact, es becomes era.

Why is explicó in the Preterite tense rather than the Imperfect?

We use the Preterite (explicó) because the action of explaining was a single, completed event in the past. The doctor made this specific statement at a specific moment, and then the action was over.

Why doesn't the adjective útil change its ending to match the feminine noun terapia?

Adjectives that end in a consonant, like útil, generally have the same form for both masculine and feminine nouns. If the noun were plural (las terapias), the adjective would change to plural (útiles), but it does not change for gender.

Why do médico, explicó, and útil all have written accents?

They follow different Spanish accent rules. Médico is stressed on the third-to-last syllable, which always requires an accent. Explicó is stressed on the final syllable and ends in a vowel, which is standard for many Preterite tense forms. Útil is stressed on the second-to-last syllable but ends in a consonant that isn't N or S, so it needs a written accent to show the unusual stress.