Dought & clear, - Reasons for doing the prostration of forgetfulness.

When is it prescribed for a worshipper to do the prostration of forgetfulness?.
Praise be to Allaah.
By the mercy of Allaah to His slaves, as one of the beauties of this
perfect religion, Allaah has prescribed that His slaves may make up
for shortcomings and mistakes that they make in their worship and
cannot avoid completely, by performing supererogatory (naafil) acts of
worship, praying for forgiveness and so on.
One of the things that Allaah has prescribed for His slaves to make up
for shortcomings that may occur in their prayer is the prostration of
forgetfulness, but it is only prescribed to make up for certain
things; it does not make up for everything nor is it prescribed for
everything.
Shaykh Ibn 'Uthaymeen (may Allaah have mercy on him) was asked about
the reasons for doing the prostration of forgetfulness, and he replied
as follows:
The prostration of forgetfulness in prayer is generally prescribed for
three reasons:
1-Doing something extra
2-Omitting something
3-Being uncertain
Doing something extra: for example, doing an extra bow (rukoo'),
prostration (sujood), standing or sitting.
Omitting something: such as omitting an essential part of the prayer,
or omitting one of the obligatory parts of the prayer.
Being uncertain: such as when a person is not sure how many rak'ahs he
has prayed, whether it was three or four, for example.
In the case of doing something extra, if a person adds something to
his prayer – bowing, prostrating, standing or sitting – deliberately,
then his prayer becomes invalid, because when he added it, that means
that he did not do the prayer in the manner ordained by Allaah and His
Messenger(peace and blessings of Allaah be upon him). The
Prophet(peace and blessings of Allaah be upon him) said: "Whoever does
an action that is not part of this matter of ours (i.e., Islam), will
have it rejected." Narrated by Muslim, 1718.
But if he did that extra thing by mistake, then his prayer is not
invalidated, but he should do the prostration of forgetfulness after
saying the salaam. The evidence for that is the hadeeth of Abu
Hurayrah (may Allaah be pleased with him) who said that when the
Prophet(peace and blessings of Allaah be upon him) said the salaam
after two rak'ahs in one of the two afternoon prayers, either Zuhr or
'Asr, and they told him about that, he(peace and blessings of Allaah
be upon him) did the rest of the prayer, then he said the salaam, then
he prostrated twice (the prostration of forgetfulness) after saying
the salaam. Narrated by al-Bukhaari, 482; Muslim, 573.
And Ibn Mas'ood (may Allaah be pleased with him) narrated that the
Prophet(peace and blessings of Allaah be upon him) led them in praying
Zuhr and he prayed five rak'ahs. When he had finished it was said to
him: "Has something been added to the prayer?"
He said: "Why are you asking that?"
They said: "You prayed five (rak'ahs)."
So he turned towards the qiblah and prostrated twice. Narrated by
al-Bukhaari, 4040; Muslim, 572.
In the case of omitting something, if a person omits one of the
essential parts of the prayer, one of the following two scenarios must
apply:
Either he remembers it before he reaches the same point in the
following rak'ah, so he has to go back and do that essential part of
the prayer and whatever came after it,
Or he does not remember it until he reaches the same point in the
following rak'ah, in which case the following rak'ah takes the place
of the one in which he omitted that essential part, and he should make
up the invalid rak'ah.
In either of these two cases, he should do the prostration of
forgetfulness after saying the salaam.
For example: a man stood up after doing the first prostration of the
first rak'ah and did not sit or do the second prostration. When he
started to recite Qur'aan he remembers that he had not done the second
prostration or sat between the two prostrations. In that case he
should go back and sit as between the two prostrations, then
prostrate, then stand up and do whatever is left of his prayer, and do
the prostration of forgetfulness after saying the salaam.
An example of one who did not remember until after he reached the same
point in the following rak'ah is a man who stood up following the
first prostration in the first rak'ah and did not do the second
prostration or sit between the two prostrations, but he did not
remember that until he sat between the two prostrations in the second
rak'ah. In this case he should make the second rak'ah the first
rak'ah, and add another rak'ah to his prayer, then say the salaam then
do the prostration of forgetfulness.
Omitting an obligatory part of the prayer – such as if he omits an
obligatory part and moves on to the next part of the prayer. For
example, he forgets to say Subhaan Rabbiy al-A'la (Glory be to my Lord
Most High) and he does not remember until he has got up from
prostrating. In this case he has omitted an obligatory part of the
prayer by mistake, so he should carry on with his prayer then do the
prostration of forgetfulness before saying the salaam, because when
the Prophet(peace and blessings of Allaah be upon him) omitted the
first tashahhud he carried on with his prayer and did not go back and
repeat anything, then he did the prostration of forgetfulness before
saying the salaam.
In the case of being uncertain, such as if a person is not sure if he
has done something extra or omitted something, e.g., he is not sure
whether he has prayed three rak'ahs or four, one of the following two
scenarios must apply:
Either he thinks that one or the other is more likely to be case,
whether it is doing more or omitting something, in which case he
should proceed on the basis of what he thinks is more likely to be the
case, complete his prayer accordingly, then do the prostration of
forgetfulness after saying the salaam;
Or neither appears more likely to be the case, so he should proceed on
the basis of what is certain, namely the lesser amount, and complete
his prayer accordingly, then do the prostration of forgetfulness
before saying the salaam.
For example: a man prays Zuhr then he is not sure whether he is
praying the third or fourth rak'ah, but he thinks it more likely that
it is the third. So he should pray another rak'ah, then say the
salaam, then do the prostration of forgetfulness.
An example of when neither seems to him more likely to be the case is
when a man is praying Zuhr and is not sure whether he is in the third
rak'ah or the fourth, and neither seems to him more likely to be the
case. In that case he should proceed on the basis of what is certain,
which is the lesser amount. So he should regard it as the third
rak'ah, then do another rak'ah and do the prostration of forgetfulness
before saying the salaam. Thus it becomes clear that this should be
done before the salaam if you have omitted one of the obligatory parts
of the prayer or if you are not sure how many rak'ahs you have done,
and neither of the two choices seems more likely to be the case. And
it should be done after the salaam if you have added something extra
to the prayer or you are not certain but one of the two choices seems
more likely to be the case.
And Allaah is the Source of strength.