ToBE #2 : Use of apostrophe

Person 1 : "I'm busy. I can't be there at 9 o'clock".
Person 2 : "Let's meet at 10 AM. I'll try to get Babu's brother too".
Person 1 : "Yes, I'd like to meet him. He was one of the first MBA's from our college".

Without dissecting the above discussion on grounds of logic, consider the use of apostrophe. One thing stands out : the apostrophe ( ' ) stands for some missing letter(s) .... most of the time.

I'm - I am. "I am ..." becomes "I'm"

can't - can not
don't - do not
shan't - shall not

o'clock - of the clock. As per the clock, the time is 7. => The time is 7, of the clock ----> shortened to ---> It is 7 o'clock.
(I think the above usage comes from the time when people moved from sundials to clocks. People had to specify that they looked at the new thing, the clock, for time and not the tested and proven sundial).

Let's meet - Let us meet. "Lets meet" is wrong. "Mother lets me play for an hour" is correct. Note that in the second sentence, lets stands for 'allows' and not 'let us'.

He'll / I'll - 'll stands for "will".

I'd - 'd stands for "had" or "would". It depends on the context.
I'd like to meet him = I would like to meet him.
I'd three marbles = I had three marbles.

Babu's - Here, it is in possessive case => "of Babu". When the last letter of the word is "s", the "s" after the apostrophe can be skipped. Examples:
Jesus' mother was Mary. (Jesus' instead of Jesus's).
The Sharma brothers' bicycle was stolen. (Bicycle belonging to the Sharma brothers was stolen).

Apostrophe-s ( 's ) does not make a word plural. There are exceptions:
Plural of abbreviations : Five B.A.'s, three M.C.A.'s. Apostrophe is not mandatory here.
Plural of numbers : There are five 5's in twenty-five.
Plural of single letters : There are two u's in "vaccuum".