Dear Rabbi Simon.
Do I fulfil my obligation for evening Shema in any way if I say it before nightfall i.e in the early summer Maarivs?
I find that in the summer once I’ve davened early Maariv with a minyan– I don’t remember to say Shema again before bed.
According to the preponderance of opinions, one who davens Maariv before sunset does not fulfil the mitzvah of the evening Shema. Even when Maariv is recited just after sunset, one needs to repeat Shema later on, after nightfall.
This is one of the reasons that some are accustomed to saying all three paragraphs with the bedtime Shema. However, it is preferable to set an alarm or other reminder for after nightfall (approximately 10 minutes before the time of Shabbos ending) and to say Shema at that time.
On Friday night (in the summer) I generally say Shema again right after birkat ha-mazon. (There is no need to do so until the meal is finished.)
Finally, although the ideal threshold for Maariv/evening Shema is shortly before the time of Shabbat ending, at Kesher I do not announce the need to repeat Shema if we daven Maariv beyond 30 minutes after sunset. IOW that is already considered dark enough to fulfil the basic requirement for evening Shema.
I hope this compound answer is helpful.
Rabbi Rashi Simon