The question asked is "What is the difference between 'whereby' and 'in which'?"
ERROR NO. 1: "It is the difference between two prepositions".
While in which can be referred to as "preposition + which", whereby is NOT a preposition. It's a conjunction or a relative adverb. If the Speak Good English Movement can't even get that right, they really ought to pack their bags and spare the rest of us their illiterate babbling.
ERROR NO. 2: "Prepositions are words that link words within a sentence".
That's totally wrong. It's at best a partial description of a conjunction, NOT a preposition. I understand that every child in Singapore who has gone through Primary 3 education knows this. Well, the Speak Good English Movement doesn't.
ERROR NO. 3: "You might say something like 'the rules by which you must follow' or 'the rules whereby you must follow'.
This is pathetic. Singapore's Speak Good English Movement is not even able to give an example of how the word whereby may be used in a sentence.
When follow carries the meaning of "conform to or comply with or obey advice, command or rules", it is always transitive. The Oxford English Dictionary quotes Iris Murdoch, "I...followed my rule of never speaking frankly to women in moments of emotion."
It is grammatically incorrect to say "the rules by which you must follow" or "the rules whereby you must follow". You simply say "the rules which you must follow" or "the rules you must follow". This is elementary and I would expect a child of three to have no problem with it.
If you would like to look at a list of all my articles on the errors of the Speak Good English Movement, MOE's language experts and other teachers of English, please visit my List of Grammar Terrorists.