BE GOING TO DO SOMETHING
1.    Intention - I haven’t done anything to make it happen.
I am going to study Spanish. (I just want to do it, I haven’t enrolled on a Spanish course)
I am going to take up jogging. (I just want to do it, I haven’t bought a tracksuit yet.)
2.    Something is going to happen based on this what I CAN SEE or KNOW
Look at those clouds. It’s going to rain. (I can see some clouds)
He doesn’t study much. He’s going to fail an exam. (I know he doesn’t study much.)
PRESENT CONTINUOUS
1.    Close arranged (planned ) future. It’s almost 100% sure that something will happen, it’s arranged between 2 parties, e.g. between me and another person or institution.
I’ve booked a flight. I am flying to Berlin next Friday. (it’s arranged between me and Finnair – I booked a flight ticket)
I’ve arranged a meeting with Jarmo. We are meeting in the evening.  (I called him and we discussed when we could meet.)
PRESENT SIMPLE
1.    Timetables, programmes etc.
The train to Leeds leaves at 7 pm. (a timetable)
The Simpsons starts at 12. (a programme)
FUTURE SIMPLE
1.    Offers, threats, promises
I promise I will do this. (a promise)
I will kill you./If you do this again, I will spank you. (threats)
Shall I help you? (I/we – shall) (an offer)
I will kill you if you don't tell me the truth. (a threat)
2.    Prediction based on NO evidence
Tomorrow it will rain. (I just think so)
3.    With expressions stating my opinion
I think/Maybe/Perhaps/In my opinion/I suppose/I hope/I believe/I am sure, I will pass my exam.
4.    First conditional sentence to talk about real present situation or possible future (If + present simple, future simple)
If you push this button, the TV set will work.
5.    Instant (quick) decision
A: There is no sugar at home.
B: I’ll buy some. (I made a decision only when I when I got to know that there was no sugar at home)
6.   Invitation, request
Will you come to my party?
Will you help me in the garden?
7. distant future
In 2050 people will fly to the moon on holiday. (in 32 years’ time – a long time from now.)

Problems
 
Will or going to?
 Will and going to are both used for predictions, and on most occasions either is possible.
 I think it’s going to / it’ll probably rain tomorrow.
 However if there is strong evidence in the present situation, then going to is usually used.
 I think it’s going to rain this afternoon, look at the back clouds in the sky.
 Going to or present continuous?
 Going to and present continuous are both used for plans and arrangements, and most occasions either is possible.
 I’m going to have / I’m having a party on Friday.
 However, going to can suggest that the detail are not yet finalized – it’s still just a plan. Whereas the present continuous can suggest that the arrangement is more fixed, with a time and place.
 Jean is going to have another driving lesson soon. (a plan, with no specific time)
Jean is having her driving lessons this afternoon. (it’s fixed in her diary)