As mentioned (above, 1:7), the Sages instituted three daily prayers: Shaĥarit corresponding to the morning Tamid offering; Minĥa corresponding to the Tamid of the afternoon; and Ma’ariv corresponding to the burning of the organs and fats upon the altar (above, 2:2-5 we learned which prayers are obligatory for women). Just as when the Temple existed every woman was permitted to bring voluntary offerings, so too, a woman may recite an additional voluntary Shemoneh Esrei. As R. Yoĥanan states: “Would that a person pray all day long” (Berakhot 21a). In order for her prayer to be recognized as voluntary, she must add some sort of special personal request in that prayer. Moreover, just as a musaf sacrifice may not be offered voluntarily, so too one may not pray Musaf voluntarily. And just as voluntary offerings are not sacrificed on Shabbatot and festivals, so too there are no voluntary prayers on those days (SA 107:1-2).
One who wishes to recite a voluntary prayer must be certain that she is careful and capable enough of having kavana in her prayer from beginning to end. If she cannot concentrate well, she is not to recite a voluntary prayer at all (SA 107:4).
Nowadays, the accepted teaching is that we do not recite voluntary prayers because we do not have the proper kavana. Nevertheless, a woman who wishes to pray Ma’ariv even though she is exempt may do so, and her prayer is not considered a tefilat nedava. Therefore, even if she is not completely sure that she can have kavana for the whole prayer, she may pray Ma’ariv and it is a credit to her.
A woman who prays Shaĥarit, Minĥa, or both on a regular basis and is not sure on a particular day whether or not she recited one of these prayers, as long as the time for its recitation has not yet passed, she recites the prayer and makes mental stipulation: “If I already prayed, this prayer is considered a voluntary prayer, and if not, this is an obligatory prayer.” It is unnecessary to introduce any personal requests into that Amida, since the fact that she is avoiding safek is itself something novel. Even though we do not recite voluntary prayers nowadays, we are permitted to do so in order to avoid uncertainty. If, in the middle of that Amida, she remembers that she already prayed, she continues to recite the Amida until the end and adds a personal request to demonstrate that it is, indeed, a voluntary prayer.
If one starts to pray on the basis that her prayer is obligatory, thinking that she did not yet pray, but suddenly remembers in the middle of her Amida that she already did, she must stop immediately. Her prayer cannot become voluntary, for just like there is no offering that is partly obligatory and partly voluntary, so too, there is no prayer that begins as obligatory and ends as voluntary (SA 107:1).
One who is daydreaming in the middle of her prayer to the point where she is unsure what part of the Amida she is reciting, for instance, one who is pondering whether she is reciting the sixth berakha or the tenth, must return to the sixth berakha and start to pray from that point on (according to most poskim, as cited in Peninei Halakha: Prayer, ch. 18 n. 3).