will do v.s. be going to do

[optimistic pessimist]

Hi all,
I learned when you talk about something that has already been planned, you say "I'm going to do",
whereas when you decide to do something on the spot, you say "I'll do..."
However, I wonder if it's a rigid rule because I remember hearing a dialogue like below.

A: What's your plane for the summer vacation?
B: I'll visit Paris with my family. / I'll probably visit Paris with my family.​

I don't remember if she said "I'll visit" or "I'll probably visit", but I'm sure she didn't say "I'm going to ...".

Don't you use "I'll" when you have already decided to do something?

OP

 

[Charlie Bernstein]

Hi all,
I learned when you talk about something that has already been planned, you say "I'm going to do",
whereas when you decide to do something on the spot, you say "I'll do..."

I've never noticed that.

However, I wonder if it's a rigid rule, because I remember hearing a dialogue like below.

A: What's your plan for summer vacation?
B: I'll visit Paris with my family. / I'll probably visit Paris with my family.

It's fine either way. Visiting Paris is what she will do.​

I don't remember if she said "I'll visit" or "I'll probably visit", but I'm sure she didn't say "I'm going to ...".

It doesn't matter.

Don't you use "I'll" when you have already decided to do something?

Yes, you do.

OP

In that dialogue, "I'm going to" might be slightly more natural than "I'll," which might be why you thought there was a rule.

Both are fine.

 

[Tdol]

However, I wonder if it's a rigid rule because I remember hearing a dialogue like below.

Few rules about speaking about the future are rigid.