It’s a nice day, so you have decided to take a walk Just before you go, you tell your friend.
You: The weather’s too nice to stay in. I’m going to take a walk.
Friend: Good idea. I think I’ll join you.
Question: Is it okay to use “I’ll take a walk” instead of “I’m going to take a walk” in this situation?
‘I’m going for a walk’ or ‘I’m going to take a walk’ first spring to mind. (First thing I think of).
Firstly, I agree with Ozemite that ‘I’m going for a walk’ is the most natural way to say it.
The use of ‘going to’ in this example just helps to emphasise the intent of the speaker. He/she doesn’t have to go for a walk, but really wants to.
will 「その場で生じた意志」——the intention that appears at the moment——
be going to 「前もって考えられていた意図」——the future action you have intended beforehand——
“Question: Is it okay to use “I’ll take a walk” instead of “I’m going to take a walk” in this situation?”
They are not necessarily interchangeable. The former could describe a situation where you made the decision exactly at that moment, and the latter describes a situation where you have had the intention beforehand. The passage “It’s a nice day, so you have decided to take a walk” shows the situation which precedes the conversation between the two people. You have had the intention before you talk about it, so you should use “be going to”. This would show the inner process of your mind clearly.
“Friend: Good idea. I think I’ll join you.”
In the above sentence, the decision to join you was made exactly at the moment when your friend said so. In that situation “be going to” cannot be used. Your friend would say, “I will join you,” responding to your statement.