Monday Mechanics: Will vs. Going To

I read that when using a future tense, “will” expresses a willingness, “going to” expresses a prior plan, and either is used for predictions…interesting and a little confusing given how often we interchange the verb…it makes me wonder if the best way to figure out which verb to use is to substitute the word “intention.”

Why did you buy the crate?  I’m going to buy a dog (as in, I’m hoping to buy a dog) versus I will buy a dog (as in  it’s my intention to buy a dog)…

To be honest, th...