The earliest time for Shema is at tsais hakochavim. On a clear night that would be 2 small stars, and on a cloudy night wait until 72 minutes after sunset. (ShA1 – MB1,4)
If you daavened Maariv before tsais hakochavim, repeat the Shema without the berachos before and after. Those berachos, you fulfilled already from after Plag Hamincha just like Maariv Shemoneh Esray. (ShA1 – ShaH6)
If you are with a tzibbur who are praying not only the Maariv Shemoneh Esray while still day, but also Krias Shema and its berachos, even though they shouldn’t be saying Shema so early, say it all with them (have in mind not to fulfil the mitzva) and repeat the Shema later. The Vilna Gaon held it’s better to daaven Maariv alone in the proper time than to daaven b’tzibbur early. (ShA1 – MB8,9 – BeH “v’im”)
Whenever you repeat Krias Shema after tsais hakochavim, at least say the first two paragraphs. The Shaagas Aryeh holds to say the third one as well. (MB11)
Immediately after shkia is considered night regarding the beracha of Hamaariv Aravim. (ShaH6)
The best procedure is to daaven Maariv b’tzibbur after tsais .If you daavened earlier, do not rely upon the Krias Shema al Hamitta. Rather, recite a separate one with kavana for the mitzva when tsais arrives. If you do rely upon your Krias Shema al Hamitta, at least make sure to say all the parshios according to the halacha and with kavana for the mitzva of Krias Shema. (MB12)
Another option is to be silent while the tzibbur recites Krias Shema and its berachos. Then daaven Shemoneh Esray with them and wait until tsais to say the Krias Shema and, its
berachos privately. This was the minhag vasikin in the days of the Chofetz Chaim (except on Friday night. See BeH end of Siman 60) (MB12 – BeH “u’mihu”)
You do not fulfil the mitzvah to be Somaych Geula L’tefilla by saying Krias Shema without the berachos. (MB15)
Do not begin even a small meal or sleep even a little bit, a half hour before Z’man Krias Shema (tsais hakochavim) – you mustn’t get carried away and lose the mitzva. Snacking on fruit or bread up to a k’baytza is allowed. (ShA2 – MB16,17)
It is a mitzva to study Torah during this time. But once tsais arrives, even learning must be interrupted in order to do Krias Shema. (MB17)
If you will be daavening Maariv with the tzibbur or you get someone who isn’t learning Torah to remind you to daaven, there is no restriction on eating or learning. (MB17,18 – ShaH19)
If you daavened Maariv after Plag Hamincha and you will need to repeat the Shema, you may begin a meal before Z'man Krias Shema. However, once the z'man comes, it is correct not to begin a meal until you repeat Krias Shema. (MB19)
If you began your meal at a permissible time, you do not have to interrupt the meal even for Krias Shema (which is d’oraisa) as long as there will be ample time after the seuda. (MB21)
If you began the seuda at a prohibited time, you need only to interrupt the meal for Krias Shema which is d’oraisa – not for its berachos nor for Shemoneh Esray. (ShA2 – MB21)
No matter when you began, if there will not be time after the seuda to daaven Maariv, you must interrupt the meal after tsais hakochavim to daaven the entire Maariv. (Rama2 – MB25)
The times for Krias Shema at night:
L’chatchila at tsais hakochavim – you are called a zariz!
You can fulfil it until chatzos and not be called a transgressor.
After chatzos you can fulfil it d’oraisa but you transgressed the word of the sages (who made chatzos a fence) unless you were delivering a public shiur (makom had’chak).
After ammud hashachar you missed the mitzvah unless it was beyond your control to say it earlier. If so, you can say it until sunrise. (ShA3,4 – MB26,27 – BeH “uz'manoh”)
Examples of “beyond your control” are a person who began to drink thinking he would say the berachos while sober before dawn, but wasn’t. Or, a person who was feeling too sick until dawn after dawn. When you say the berachos after dawn, omit the paragraph Hashkivaynu and Boruch Hashem L’olam. (ShA4 – MB31,32,34)
Last chance to say Shemoneh Esray of Maariv, in any case, is until dawn. (MB34)