Hola Dennis

 I imagine this is the question you are referring to as it is the last one you did in your tests:

Ustedes ________ lo que pensaban.

You said what you were thinking.

(HINT: Conjugate "decir" in Pretérito indefinido)

It is indeed saying "ustedes" in the Spanish sentence, and "you" in the translation, but this is because all pronouns "usted, ustedes, tú, vosotros" are translated as "you" in English.If you see that it is "ustedes" (you plural) and not "usted" (you singular), then you need to use the verb in the plural, "dijeron". It's only when the subject is not explicit in the question that we alert on which one to conjugate it for, e.g. "Conjugate in the you plural formal form" or "Conjugate in the you singular informal form" etc. When it is explicit, we don't need to add the hint as it's clear from the text. 

I hope this clarified it for you.

Un saludo cordial

Inma