Learn French: Il ne faut pas se fier à cette prévision.

Il faut (and its negative il ne faut pas) is an impersonal expression meaning "it is necessary" or "one must/must not". While on ne doit pas is also correct, il ne faut pas is the most natural and common way in French to express a general rule, warning, or piece of advice that applies to everyone.

Why does the verb se fier have the pronoun se in front of it here?

The verb meaning "to trust" or "to rely on" is se fier, which is a pronominal verb. It always requires a reflexive pronoun. Because Il ne faut pas doesn't refer to a specific person (like "I" or "you"), we use the default third-person reflexive pronoun se before the infinitive.

Do I always have to use the preposition à with se fier?

Yes, se fier is always followed by the preposition à when introducing the person or thing you are trusting. You cannot translate the English "trust this forecast" directly; in French, you "trust to" something, which is why we say se fier à cette prévision.

Since this lesson is about reported speech, how would I report this sentence if it had originally been spoken in the future tense?