The use of be going + infinitive and the present continuous tense to speak about the future is similar. We use them to talk about things that are already planned or decided.
|What are you doing this weekend? (= What are your plans?)|
|I'm spending the weekend at home. (= I've planned it already/I've already decided)|
|I'm going to spend the weekend at home. (= I've already decided/I've planned it already)|
The present continuous is usually used to speak about personal arrangements, when the time and/or place have already been decided.
|We're meeting Jim at the pub at 6 o'clock.|
|The taxi's picking us up at 3 o'clock.|
Be going to + infinitive is used to talk about our intentions, even if they are some way in the future.
|I'm going to study law when I finish school.|
|We're going to Hawaii for our holidays next summer.|
Notice that last sentence - it is not common to say "going to go to. We normally drop the go to and just say going to.
Be going to + infinitive is also used to speak about the future when we have already got some evidence that something is certain or likely to happen.
|She's going to have a baby (I can see that she is pregnant)|
|It's going to rain (I can see the dark storm clouds)|
Will is more complicated, and here we are only going to discuss its use in relation to be going to + infinitive and the present continuous.
In certain situations all of these future forms can have similar meanings. The difference is what we want to stress when we speak. As we wrote above, be going to + infinitive and the present continuous are used to talk about things that are already planned or decided. If this is what we want to stress then we would use one of those structures, if not, we would choose will:
|What are you going to do now?|
|What are you doing now?|
|What will you do now?|
In the first two examples, the person asking the question assumes that the person they are speaking to has some plan or intention. In the last example, s/he feels that there is some uncertainty.
Also above, we mentioned the use of be going to + infinitive to talk about (predict) the future when we have already got some evidence that something is certain or likely to happen. When this evidence is not present, or at least is not as concrete, we prefer to use will.
|It's going to rain later (I can see the clouds building up)|
|I think it will rain later (It often does at this time of year)|