Fortuna adversa amicis fidendum non est.

In Orberg LLPSI he has: Fortuna adversa amicis fidendum non est.

I can’t figure out the syntax and grammar - or meaning. Fidendum is gerundive right?
So it’s something like : adverse fortune has no friends??

Fortunâ adversâ = in [times of]/with adverse fortune
fidendum non est = [impersonally/impersonaliter] there should not be placed trust in/one should not trust in… [this verb takes a dative and the gerundive can keep this case for its object // hoc verbum dativo casui servit quo in casu gerundivum objectum tenere potest].

Adrianus

Gratias tibi ago.

Paul

Shouldn’t that be fidenda, to agree with fortuna?

Fortuna is ablative, not the subject. The subject is impersonal.

Adrianus, isn’t an alternative explanation that ‘fidendum’ doesn’t take dative but that ‘amicis’ is dative as it is an agent (is this the same thing?). So that it means: in times of adverse fortune friends are not trustworthy. ‘amicis’ - dative - agent.?

Orberg explains this in notes to the previous chapter: 'With the gerundive, which is a passive form, the dative (not ab + abl) is used to denote the agent, i.e. the person by whom the action is to be performed: Quidquid dominus imperavit servo faciendum est.

Or is this (agent) only referring to cases where someone asks or orders someone else to do something…?

The verb “fideo” DOES take the dative but, COINCIDENTALLY, that’s the way gerundives work impersonally.
“Fido” verbum quidem dativo casui servit quod, casu, gerundivum impersonaliter facit.
“There/it should be [done] to a thing” = “A thing should be [done]”
Here the [indirect] object is the agent.
Hîc objectum [indirectum] est quod agit.

Adrianus,

Quando dixisti: 'fidendum non est = [impersonally/impersonaliter] ’ Nonne ita neutrum ‘fidendum’ explicatur.

Paulus

It’s neuter because it’s impersonal, or vice versa, if you prefer.
Neutrius generis quod impersonale; vel vice versâ, si velis.