Learn French: Les conseils qu'il a donnés disent qu'il faut que je mincisse. - The advice that he gave says that I must lose weight.

The verb is mincir (to lose weight), which is a regular -ir verb. To form the subjunctive, you take the present tense "ils" form, which is ils mincissent. You drop the -ent to get the stem minciss-, and then add the "je" subjunctive ending -e, resulting in que je mincisse.

What triggers the subjunctive mood here?

The expression il faut que (it is necessary that / one must) expresses obligation or necessity. This is one of the most common triggers for the subjunctive in French, so the verb that follows it must be in the subjunctive.

Why does donnés have an 's' on the end? Doesn't the passé composé with avoir usually stay the same?

Usually, verbs conjugated with avoir do not change endings. However, there is a big exception: if the direct object comes before the verb, the past participle has to agree with it in gender and number. Here, the object is qu' (representing the masculine plural noun les conseils), so we add an s to donné.

Why is les conseils plural if the English translation just says "advice"?

In English, "advice" is an uncountable noun (you cannot say "an advice" or "advices"). In French, un conseil is a countable noun meaning "a piece of advice". When referring to a collection of tips or guidance, French naturally uses the plural les conseils.