when is the earliest time to start praying Ma’ariv?

Answer: The proper time to pray Maáriv is 'tzet hakochavim'', meaning when the stars first appear. (Mishna Berura 233:9). If you have a good reason which is making it necessary to daven earlier you can start from sunset, called in Hebrew שקיעה (see Or Letzion part 2, 15:6). If there is a very strong reason that you would like to say Maariv even earlier you may daven from 'plag hamincha' which is about an hour and a quarter (75 minutes) before the stars come out. If you need that option make sure that you daven Mincha before plag hamincha. Therefore, if you need to take this leniency, you can always daven Maariv from the plag and always daven mincha before the plag. If you do this permanently than it is considered l"chatchila. However, if you habitually daven Mincha after the plag then davening Maariv after plag would only be allowed if you have a special need to do it. (see Mishna Berura 233:6) For detailed and clear explanations about all this you can read Peniei Halacha in English on the internet (Tefila, chapter 25, 5-9) You can see on the listings of the exact time of tzet hakochavim, shkia and plag, daily. All the best