Answered by Shaykh Faraz Rabbani
Historically, I did not know that I needed an intention for a Ramadan fast. I did not know if I made an intention for each fast that I did. I think I knew it was fard. Can I assume that an intention was made for each fast? If not, can I just do one substitute fast for each one that I think I missed?
1. It is normally inconceivable that one would fast without having an intention, in the Hanafi school, as explained below:
2. The intention for fasting is very simple:
It is to know in your heart that you will fast that day. [al-Fatawa al-Hindiyya, quoting al-Khulasa]
It is valid to have this intention any time from maghrib the night before up to the Islamic midday of the actual day of fasting, for current Ramadan fasts and voluntary fasts. [The Islamic midday is half way between the beginning of fajr and maghrib time.] [al-Fatawa al-Hindiyya]
So even in the exceptional case when one did not have the determination to fast the next day at night, it would be sufficient if one had it after fajr before midday.
It is even explained that the very intention to get up for the pre-dawn meal (suhur) is an effective intention to fast. [Mentioned by Abu Bakr al-Haddadi in his Jawhara al-Nayyira, quoting Najm al-Din al-Nasafi, and in al-Fatawa al-Hindiyya; and by Ibn Abidin in Radd al-Muhtar, quoting al-Dhahiriyya via Ibn Nujayms al-Bahr al-Raiq]