Earliest time for shema is when Kohen eats his terumah (nightfall)

Latest time is end of 1st watch, (r Eliezer) or midnight (rabbis) or dawn (r gamliel)

R gamliel emphasizes his opinion in a story to his children that really one can read shema until dawn and it’s jsut preferable to take care of it by midnight

We start learning about night shema bc either the pasuk starts with the night time shema or the worlds creation begins with night

The kohen who was Previously tamei has a three step processs for purity. He immerses and may eat ma’aser but not terumah. After nightfall he may eat terumah but not kodshim. The next day he brings Korban and eats kodshim.

There are various other opinions regarding the earliest start time for the night Shema. They range from sundown, ppl eating meal Friday night, moments b4 stars emerge, emergence of stars, when poor man eats dinner, and when regular ppl eat dinner.

There is a dispute about the length of bein hashmashos (when we are unsure whether it is day or night) Acc to r Yehudah this is a significant amount of time but acc to r Yosi it is simply the blink of an eye before the stars emerge.