"He reducido la cantidad de azúcar."
Traducción:I have reduced the amount of sugar.
this a mistake made by duolingo, he has reduce the amount of sugar is correct.
He has reduced the amount of sugar = (Él) ha reducido la cantidad de azúcar.
the sentence says he that is the third person in singular for this reazon I write has
"He reducido" means "I have reduced." First person singular, not third.
Por qué no es valido: "I have reduced the sugar amount"
Doy dos lingotes al que me explique donde está mi error.
LOL, it's one of those things that native English speakers know, without being able to explain why. You can say "I have reduced the amount" but as soon as you say what it's an amount of, you have to say "I have reduced the amount of [whatever]. It just is what it is.