Learn Italian: Avevo capito di avere torto su tutto. - I had realized I was wrong about everything.

When the subject of the main verb (avevo capito) is the same as the subject of the second verb, Italian prefers to use di + the infinitive (avere) instead of che + a conjugated verb. While Avevo capito che avevo torto is technically understandable, the di + infinitive structure is much more natural and widely used.

How does avevo capito relate to the tense backshifting we are learning?

Avevo capito is the trapassato prossimo (past perfect), translating to "I had understood" or "I had realized." If someone originally said or thought something in the passato prossimo (Ho capito - "I realized"), that tense shifts one step further back into the past (avevo capito) when you report it later as part of a past-tense narrative.

Why is the verb avere (to have) used to mean "I was wrong"?

In Italian, you do not "be" wrong, you "have" wrong. The idiom is avere torto (literally "to have wrong"). Its exact opposite, "to be right," follows the exact same pattern: avere ragione.

Why use the preposition su before tutto?