As for additions in actions, if they are unrelated to the prayer, it has previously been mentioned that they fall into five categories: movements during the prayer. (1)

If they are of the same nature as the prayer, then:

If they alter the form of the prayer—namely, bowing, prostrating, standing, or sitting—then the prayer is invalidated if this is done deliberately; otherwise, it is not invalidated, and he should perform the prostration of forgetfulness.

If they do not alter the form of the prayer, such as when one raises his hands level with his shoulders at a point other than where raising them is prescribed, the prayer is not invalidated by this, because it does not alter the form of the prayer. However, according to the stronger view, the prostration of forgetfulness is prescribed for him.

Whoever omits a pillar and remembers it after having begun the recitation of another rak'ah, the rak'ah in which he omitted it becomes void, …

His statement, “Whoever omits a pillar,” means: when he omits a pillar. The pillars have already been explained. (2) If it is the opening takbīr, his prayer has not commenced, whether he omitted it deliberately or forgetfully, because the prayer does not commence except with the opening takbīr. Thus, if a person stood intending to pray, forgot to say the takbīr, began the opening supplication, recited al-Fātiḥah, and continued, we would say that his prayer never commenced in the first place, even if he prayed all the rak'ahs. If it is a pillar other than the opening takbīr, then it is what the author, may Allah have mercy on him, mentioned.

His statement, “and remembers it after having begun the recitation of another rak'ah, the rak'ah in which he omitted it becomes void”: “becomes void” means that it becomes disregarded, not void in the sense that is the opposite of validity; for if it meant void in the sense that is the opposite of validity, it would be necessary