A question from Anna in the Netherlands:
What's the difference in meaning between 'will' and 'going to'. So should I say for instance 'I will go the market at four' or should it be: 'I'm going to the market at four?'
Based on your actual examples,
'I will go to the market at 4' and 'I'm going to the market at 4',
I think it's actually worth looking at 3 possible verb forms for the future:
Will, going to do, and finally, in the case of your example, I'm going.
So let's start by looking at how we use will and the bare infinitive, as in your example 'will go'. This verb tense is known as the future simple, and has several functions. One of the most common is to express a prediction, that is a guess or a subjective opinion, about the future, when we've not made any definite arrangement, but just think that something is probable, or likely to happen at the time of speaking.
If I asked you the question: who do you think will win the World Cup? I'm guessing that you (Anna) might say 'I think Holland will win', and you might also feel really confident about that. But I think even the most passionate football supporter would agree the final result (of a football tournament that hasn't yet finished) can't be thought of as definite or something that has been arranged in advance.
In a similar way, we also use 'will' for decisions, offers, promises or threats that are spontaneous, or made quickly at the moment of speaking. The speaker hasn't decided before. If you saw the sun was shining outside, you might say
'It's a beautiful day, I think I'll, or I will, go for a swim later,' or 'maybe I'll phone my friends and organise a picnic'.
In all of these situations, the common link is that there are no definite arrangements for these events. No decision has been made before speaking.
Moving on now to a different structure: Going to + bare infinitive, which is sometimes used quite interchangeably with 'will'. This has a particular function for stronger predictions, perhaps when there's some present evidence to suggest something will happen: 'Ella's a really good student, I think she's going to be a brain surgeon when she gets older.'
We also use this structure when we have a personal intention, or are making a resolution or decision to do something, as in, 'I'm going to stop eating so much chocolate this year'.
In a context where you have not only decided to do something but also made all the arrangements, sometimes referred to as 'diary future', we're more likely to use a present verb tense, the present continuous ? am/is/are + ING form of the verb, as you have in your second sentence 'I'm going to the market at 4'. We use this form for future events that are booked and already arranged, and which we consequently feel are definitely going to happen. For example, I'm flying to France on Sunday, we're buying a house or I'm meeting my boss at 2.
So while there's often a confusion between these forms I hope those examples have helped to clarify the key difference for you. Thanks Anna.