✅ Answer: There are different scenarios for the above question:

1. A person who completely and absolutely forgets that they have a vow. In this case, pay attention to the following two narrations:

1 حَرِيزِ بْنِ عَبْداللَّه عَنْ أَبِي عَبْداللَّه قال: قَالَ رَسُولُ اللَّهِ رُفِعَ عَنْ أُمَّتِي تِسْعَةُ أَشْيَاءَ الْخَطَأُ و النِّسْيَانُ وَ مَا أُكْرِهُوا عَلَيْهِ و مَا لَا يَعْلَمُونَ وَ مَا لَا يُطِيقُونَ وَ مَا اضْطُرُّوا إِلَيْهِ وَ الْحَسَدُ وَ الطِّيَرَةُ وَ التَّفَكُّرُ فِي الْوَسْوَسَةِ فِي الْخَلْوَةِ [الخلق] مَا لَمْ يَنْطِقُوا بِشَفَةٍ

📚 Al-Tawhid, p. 353, hadith 24

Imam Sadiq (peace be upon him) said: The Messenger of Allah (peace be upon him and his family) said: 'Nine things have been lifted from the Ummah:

Mistake, forgetfulness, and that which they are forced into, and that which they do not know, and that which they do not have the capacity for, and that which they are in dire need of, and envy, and bad omens, and thinking about whispers in creation as long as (in these three cases) they do not utter a word!'

2 عمرو بن مروان عن أَبِي عبداللَّه قال: قَالَ رَسُولُ اللَّه رُفِعَ عَنْ أُمَّتِي أَرْبَعُ خِصَالٍ خَطَؤُهَا و نِسْيَانُهَا و مَا أُكْرِهُوا عَلَيْهِ و ما لَمْ يُطِيقُوا وَ ذَلِكَ قَوْلُ اللَّهِ عَزَّ وَ جَلَّ رَبَّنا لا تُؤاخِذْنا إِنْ نَسِينا أَوْ أَخْطَأْنا رَبَّنا وَ لا تَحْمِلْ عَلَيْنا إِصْراً كَما حَمَلْتَهُ عَلَى الَّذِينَ مِنْ قَبْلِنا رَبَّنا وَ لا تُحَمِّلْنا ما لا طاقَةَ لَنا بِهِ وَ قَوْلُهُ إِلَّا مَنْ أُكْرِهَ وَ قَلْبُهُ مُطْمَئِنٌّ بِالْإِيمانِ

📚 Al-Kafi, Vol. 2, p. 462

Imam Sadiq (peace be upon him) said: The Messenger of Allah (peace be upon him and his family) said: 'Four qualities have been lifted from my Ummah: their mistake, their negligence and forgetfulness, and that which they are forced into, and that which they do not have the capacity for.'

And this is the saying of Allah (in Surah Al-Baqarah, verse 286) where He says:

'Our Lord, do not hold us accountable if we forget or make a mistake.

Our Lord, and do not place a heavy burden upon us as you placed it upon those before us!

Our Lord, and do not place upon us that which we do not have the capacity for!'

And the saying of Allah (in Surah An-Nahl, verse 106) where He says:

'Except for one who is forced while his heart is at ease with faith.'

✅ According to the two above narrations and their likes, if a person had a vow to fulfill and they completely forget their vow, to the extent that they do not know that they had a vow at all, their Lord will not hold them accountable, and forgetfulness and negligence have been lifted from this Ummah.

2. The second scenario is when they know they have a vow but do not know the exact subject or object of it:

For example, they do not know whether they vowed to perform 5 Ashura recitations or 10?

Did they vow to give 100,000 Tomans in charity or 150,000 or 200,000 and so on?

Did they vow to perform the Ja'far prayer or to send blessings (Salawat)?

Did they vow to fast for three days or to complete one full recitation of the Quran or to give 10,000 Tomans in charity or to perform three Ashura recitations?

and so on...

✅ In cases like the above, one should act upon the following narrative principle and its likes:

3 عنوان الْبَصْرِي عن أَبِي عَبْداللَّه جعفر بن محمد يقول فِيهِ سَلِ الْعُلَمَاءَ مَا جَهِلْتَ و إِيَّاكَ أَنْ تَسْأَلَهُمْ تَعَنُّتاً و تَجْرِبَةً و إِيَّاكَ أَنْ تَعْمَلَ بِرَأْيِكَ شَيْئاً و خُذْ بِالاحْتِيَاطِ فِي جَمِيعِ أُمُورِكَ ما تَجِدُ إِلَيْهِ سَبِيلًا

📚 Wasail al-Shia, Vol. 27, p. 171

Imam Sadiq (peace be upon him) said to العنوان البصري (Al-Unwan al-Basri): '...And beware of acting upon anything from your own opinion, and in all your affairs, act with caution wherever you find a path to caution!'

✅ Therefore, according to the principle of caution, when we are in doubt about the type or amount of our vow, we perform all the options out of caution to be certain that we have fulfilled our duty.

3. In a scenario where we know that we have a vow, but we have completely forgotten its subject:

In such a case, according to narrations 1 and 2 and their likes, we are not held accountable for this forgetfulness, but since we know that we have a vow, and unlike the second scenario, we are not in doubt between several options to perform them all out of caution, we give the expiation for the vow according to narration 3, which is the same principle of caution, because in such a case, no alternative has been conveyed to us from the Ahl al-Bayt (peace be upon them) and we have no way to fulfill our vow.

In a case where the probable options are so many that we do not have the capacity to perform all of them, since matters beyond our capacity have been lifted from us according to narrations 1 and 2 and others, we suffice with giving the expiation for the vow out of caution.

✅ In the above scenarios, due to the absence of a specific text, one acts upon the general principles conveyed by the Ahl al-Bayt (peace be upon them).