"changing to informal"

Question

Ya se habia marchado.(he had already left).Would informal be Ya te habias marchado.

Answer 1

No. There is no "informal" mode for the third person. "Ya te habías marchado" is the informal mode of "Ya (usted) se había marchado" - "You had left already". The "informal" is only used for addressing the second person directly.

Tú or Vos vs. Usted.