If the worshipper forgets the first sitting and starts to stand:
• if not yet fully upright, return to the sitting;
• if already fully upright, do not return in the normal Fard case — continue and perform Sujud al-Sahw.
The commentary gives a finer distinction:
• if one returns while still closer to sitting, no Sahw is required for that return;
• if one had risen closer to standing before returning, Sujud al-Sahw is required.
The matn gives a special return rule for Nafl; the commentary records further juristic preference, so that advanced detail should not be oversimplified into the Fard rule.