at the beginning vs. in the beginning

[Englishlanguage]

Hello,
yesterday I came across a website which provides some on-line exercises. I tried one and when evaluating I realized the computer had corrected at the beginning with in the beginning. Now I wonder: is there any differences between them?
I add another question concerning the same matter. What about in the end vs. at the end?
I think it should be
In the end everythin was ok
At the end of the story, they get married.
but I'd like to have a confirmation about its usage.
Thank you

[Englishlanguage]

That's it exactly.

In the end is self-contained and refers to an abstract end, whereas at the end needs to be at the end of something.

In the beginning is pretty biblical (I can't think of anywhere I've heard it other than the book of Genesis).

So as for the meaning in the beginning and at the beginning are the same, aren't they?

[engee30]

So as for the meaning in the beginning and at the beginning are the same, aren't they?

Let me show you the way I was taught the four similar, but different in meaning, phrases:

In the beginning (+ nothing else) = At first; in the first place
In the end (+ nothing else) = Finally; at last

In the beginning, I didn't like my previous job, but I got to like it in the end.

At the beginning (of something) = At the point that something starts
At the end (of something) = At the point that something finishes

At the beginning (of the film), I was yawning, but at the end (of it) I got really interested, and my jaw dropped open!
___________________________
NOTE:
Bear in mind I'm not a teacher!

[Englishlanguage]

Let me show you the way I was taught the four similar, but different in meaning, phrases:

In the beginning (+ nothing else) = At first; in the first place
In the end (+ nothing else) = Finally; at last

In the beginning, I didn't like my previous job, but I got to like it in the end.

At the beginning (of something) = At the point that something starts
At the end (of something) = At the point that something finishes

At the beginning (of the film), I was yawning, but at the end (of it) I got really interested, and my jaw dropped open!
___________________________
NOTE:
Bear in mind I'm not a teacher!

thank you
Whether this rule is followed in common language or not, it's very easy to fix ideas