GIVE ANSWERS - English

Similar and surely confusing, but different. To make it easier, think about it in the present tense.
GOING TO is used to express something you will do in the future and it gives the impression that it’s something you planned on doing.
ex. I am going to watch football tonight.

ABOUT TO is used to express an action that is about to start in this specific moment. So while you could say “I am going to watch football now”, “I’m about to watch football” gives off the sensation that you all set up on your comfy couch and about to press play!

While using this in the past tense, you keep that same meaning, but it obviously becomes slightly more tricky: for when it comes to both you should always link it to another action, otherwise you will confuse who listens to you because the sentence feels incomplete.
“I was going to watch football but I changed my mind”
“I was about to watch football when my dog jumped on the couch and scared me”.

Anyways, GOING TO indicates that you planned something, and ABOUT TO shows that you are going to do it in the exact moment you are referring to.

To my understanding: I was going to refers to something that was planned a bit longer time ago and it was more stable. 

For instance: I was going to watch "Friends" that night. 

I was about to do something is a little bit more messy.  It would mean that I was going to do something,but something interrupted me.

I was about to leave home,but a phone call had stopped me. 

P.S. I can be wrong

”I was going to watch TV” - a plan at some point in the past existed to watch TV. Unless we have other context, we don’t know it the action of watching TV happened or not.

”I was about to watch TV” - a plan existed at some point in time in the past to watch TV soon after that point in time. However, the plan did not happen.

Note that this contrasts with ”I am about to watch TV”. Here, there is a plan to watch TV and this plan will happen soon after the statement ”I am about to watch TV” is made.

They are very similar but not exactly the same. Even though you can use one or the other for certain cases, about to, means you were going to do something and something happened and you couldn´t do what you were going to do (note that I am using going to. As I previously explained, you can use both in some cases), or you changed your mind almost instantly and you stopped.

”They’re pretty similar, but not exactly alike. ’About to’ means you were just going to do something, but then something happened and you couldn’t do it. Or maybe you changed your mind right away and didn’t do it after all.”Maharashtra Board Solutions

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