special case? (a most vs the most)

[Nessie]

Hi, please have a look at this:

Last year I had ... most enjoyable experience when I met a new friend
a. a b. the

==>> The answer in the book is "a", but I wonder why it's not "the" (superlative comparison) I once asked my teacher and he said it was a special case, but I forgot to ask him how special it was, and in which other cases we have to choose respectively.
So please shed light on this and give me some more examples.

Many thanks in advance.
Nessie.
_________________
... something we never have again, I know... I guess I really really know..

Sorry seems to be the hardest word...

[Jamie (K)]

It's not a special case. If you look down, down, down in the dictionary definitions of "most", you will find that the word can also mean "extremely" or "very". That's when we can use the indefinite article with it.

[Nessie]

Thanks, Jamie

Now I just have one more query: in this sentence, why can't we choose "the" as a superlative comparison?

Many thanks
Nessie
_________________
... something we never have again, I know... I guess I really really know..

Sorry seems to be the hardest word...

[Jamie (K)]

[Nessie]

Er... what about this:

- This is my most wonderful trip
(compared with other trips)

so why can't we say:
- Last year I had the most enjoyable experience when I met a new friend
(compared with other experience)
_________________
... something we never have again, I know... I guess I really really know..

Sorry seems to be the hardest word...

[Jamie (K)]

You can say it, but you're confusing two different usages, and two different meanings.

[Nessie]