For most informal sentences about the future, we can use the construction [ “am” / “is” / “are” ] + [ “going to” ].
Deciding at the moment of speaking
- I’m hungry. I think I’ll cook something. ✔︎
- I’m hungry. I think I’m going to cook something. ✔︎
- I’m hungry. I think I cook something. X (wrong)
- I’m hungry. I think I’m cooking something. X (wrong)
Prediction
- Look at those clouds! I think it’s going to rain. ✔︎
- Look at those clouds! I think it will rain. ✔︎
- Look at those clouds! I think it rains. X
- Look at those clouds! I think it’s raining. X (Wrong, unless it is already raining now.)
Something already planned
- We can’t come because we’re going to meet my Mum this evening. ✔︎
- We can’t come because we’re meeting my Mum this evening. ✔︎
- We can’t come because we’ll meet my Mum this evening. X (This sounds wrong in an informal sentence.)
- We can’t come because we shall meet my Mum this evening. X
Timetables
- The train arrives at 6:31. ✔︎
- The train is going to arrive at 6:31. ✔︎
- The train shall arrive at 6:31. X
Exception 1
For future meaning in formal language, we avoid “going to”, and “-ing” in general. We can use “will” for most formal sentences about the future.
- We predict that interest rates will peak in the year 2021. ✔︎
- The meeting will take place on Monday the 23rd. ✔︎
- The meeting takes place on Monday the 23rd. ✔︎
Exception 2
For “first person” questions which allow the answerers to decide what the askers do, we use “shall”.
- Shall I take your coat? ✔︎
- Am I going to take your coat? ?? (The question appears to ask the coat wearer to predict the actions of the other person.)
- Shall we order a taxi? ✔︎
- Shall I be able to find the office? X (The answerer can’t control this ability.)
Exception 3
With modal verbs, we often have to change the modal verb, if we wish to use future forms with [ “am” / “is” / “are” ] + [ “going to” ]. It is always possible, and easier, to use the bare modal verb.
- I can come tomorrow. ✔︎
- I’m going to can come tomorrow. X
- I’m going to be able to come tomorrow. ✔︎
- I must go to London tomorrow. ✔︎
- I’m going to must go to London tomorrow. X
- I’m going to have to go to London tomorrow. ✔︎
- I ought to pay my tax soon. ✔︎ (This is difficult to rewrite with future forms.)
- I’m going to ought to pay my tax soon. X
Exception 4
We don’t normally use future forms after “if” and “when”. We use the present simple.
- When I’m going to play tennis later today, I’m going to feel tired. X
- When I will play tennis later today, I will feel tired. X
- When I play tennis later today, I’m going to feel tired. ✔︎
- If I’m going to play tennis today, I’m going to get tired. X
- If I play tennis today, I’m going to get tired. ✔︎
More advanced learners
We can use “if” + [be] + “going to” to talk about the necessary conditions, rather than the results, of the main future action.
- If I’m going to play tennis today, I need to find the right clothes for it. ✔︎
Even more advanced learners: there is also an “unlikely future” version of the construction immediately above:-
“If I were going to win £1,000,000 on next Saturday’s lottery, I would need to have paid for the correct type of lottery ticket.” ✔︎ (Compare “If I won £1,000,000 on next Saturday’s lottery, I would be able to drink champagne every day.” ✔︎ )