Future Simple (I will work / Will he work? / The won’t work) is used for PREDICTIONS (προβλέψεις), WILLINGNESS (προθυμία), SUDDEN DECISIONS (ξαφνική απόφαση), PERSONAL OPINION (προσωπική γνώμη) especially with the expressions I THINK, I BELIEVE, IN MY OPINION, …
BE GOING TO + VERB (I’m going to work / Is he going to work / They are going to work) is for LOGICAL PREDICTIONS (λογική πρόβλεψη – που κάπου στηρίζεται), and PLANS / INTENTIONS (σχεδια / προθέσεις = πράξεις που έχουμε αποφασίσει να πραγματοποιήσουμε)
Present Perfect (I have finished / Has he finished ? / They haven’t finished) : is used for experiences we have already have or haven’t had yet.
OR : for activities that started time ago and haven’t finished yet (already, just, yet, since, for,…)
Past simple: I played / Did he play? / She didn’t play : is used for activities we did in the past. They are finished and completed (yesterday, last Monday/week/month/year, when…, in 2000, etc)
Example : If you tell me a story, I will give you a kiss!
If they don’t come back by 9:00, I will call the police!