How can one distinguish the true dawn (al-Fajr al-Sadiq) in order to accurately determine the time for prayer and for refraining from food and drink (imsak), and is one sinful if they err in its estimation?

Dawn is of two types: a false dawn, with which the time for Fajr prayer does not begin, nor does it prohibit fasting; and a true dawn, with which the time for Fajr prayer begins and which prohibits fasting. This is what is meant by the Almighty’s saying: (And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]).

The false dawn is an elongated whiteness in the sky that then darkens, while the true dawn is a whiteness spread across the horizon that increasingly brightens and is connected to the horizon. No legal ruling is based on the false dawn.

Calendars are not a reliable source for knowing the time of dawn, and the correct time must be ascertained by observing the differences between the two dawns. If you are unable to do so, a precautionary time can be set after the calendar's adhan, and one should abstain from food and drink before that time as a precaution.

One may eat and drink until certainty of the true dawn’s appearance when fasting.