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I've researched online and I've read that ascorbic acid can both promote oxidation and reduction depending on the conditions (I've read about trace metals but I've never found which metals affect its chemistry and which ones don't).
So if I mix ascorbic acid with a bunch of different aldehydes, will it react with all if them? Some of them? Will it reduce them (into alcohols?) or oxidate them (into carboxylic acids)? Do metals have any influence on what happens? And if so, which metals do?
You would need very strongly reducing compound to be able to reduce ascorbic acid. Aldehydes are not the case.
Ascorbic acid would not reduce aldehydes like ethanal either, as can be seen by comparison of their respective redox potentials below:
Transition d-orbital metals with multiple possible oxidation numbers very frequently act as redox catalysts. Both neighbour oxidation numbers may be common ( $\ce{Fe^{+III}/Fe^{+II}}$, $\ce{Mn^{+III}/Mn^{+II}}$, $\ce{Mn^{+IV}/Mn^{+III}}$ ), or one may be uncommon ($\ce{Cu^{+III}/Cu^{+II }, Ag^{+II}/Ag^{+I}}$)
Catalytic effect is often related to the opposite charges of reagents and catalysts. The typical example is reaction of oxalate and permanganate anions, catalyzed by manganese cations.
A good metal catalyst has standard redox potential comparable with respective redox potentials of the oxidant a/o reductant.
E.g $\ce{Ag+}$ may act as a catalyst during oxidation by peroxodisulphate, as $\ce{Ag^2+/Ag+}$ has the similar redox potential. Similarly, $\ce{Mn^2+}$ ions act as the catalyst during oxidation of oxalic acid by permanganate in acidic solutions, as $\ce{Mn^2+/Mn^3+}$ has similar redox potential as $\ce{MnO4-/Mn^2+}$.
