"I'm always meeting..." VS "I always meet..."

Try this;

I _____________________ my students at shopping malls during the holidays.
A) am always meeting
B) always meet

The answer is A.

Why?????

Because I mean that I'm always bumping into (meeting accidentally) my students whenever I visit shopping malls.

HOWEVER, if I say "I always meet my students at shopping malls during the holidays", that means I always make arrangements or plans to meet them.

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We use "always VERB+ing" (not "always VERB")  for certain situations;

1) to mean 'very often', perhaps more often than we expected.
2) something that is unexpected or unplanned or not regular.
E.g. :
I'm always losing my keys. (very often)
My pupils are always giving me presents. (very often+unexpected)

We don't use "always VERB+ing"  for something that's regular or planned.
E.g:
I always meet my students at school. (regular/planned arrangement)
We always spend our holidays together. (regular + planned)

"MUSTN'T" or "CAN'T"? (in expressing improbability)

He's a nice boy. It can't be him stealing the pencil! - CORRECT

He's a nice boy. It musn't be him stealing the pencil! - WRONG


*Must is NOT often used to express certainty in NEGATIVE clauses.
It is used to express the conclusion that something is certain or highly probable.

E.g.,
He's a bad boy. It must be him stealing the pencil! - CORRECT!