Laid her hand on her head - To hold on the ashes (see the marginal references).
Went on crying - i. e. “went away, crying out as she went.”
Matthew Henry
Concise Bible Commentary
From henceforward David was followed with one trouble after another. Adultery and murder were David's sins, the like sins among his children were the beginnings of his punishment: he was too indulgent to his children. Thus David might trace the sins of his children to his own misconduct, which must have made the anguish of the chastisement worse. Let no one ever expect good treatment from those who are capable of attempting their seduction; but it is better to suffer the greatest wrong than to commit the least sin.
SDA Bible Commentary (limited)
Volume 1-7
19. Rent her garment. This was probably done immediately. Tamar made no attempt to cover up the shame that had come upon her. She was a virtuous young woman whose conduct was entirely above reproach. As she left the apartment of Amnon she gave vigorous evidence of the deep grief she experienced within (see Esther 4:1; 2 Kings 5:8). Thus she prevented Amnon from inventing the tale that she had been guilty of misconduct toward him and for this reason had been expelled from his presence. Tamar was evidently entirely sincere, her actions betokening the keen indignation and grief that were hers. Had she kept quiet she might have been considered a party to the crime.