ajaypatil_am wrote:Hi,
When to use Unlike ....Like...As ?
As per my understanding,
Use 'As' when you are comparing two or more clauses
Use 'Like' when you are comparing two or more nouns
Unlike is confusing ...Can anyone please advice on usage of 'Unlike'

first, be aware that there are different types of 'as'. when 'as' is used by itself, you are correct: it is attached to clauses, not to nouns.
HOWEVER,
do NOT extend that reasoning to the 'as' that forms part of expressions such as 'as large as', 'as many as', etc. those expressions can perfectly well be used with nouns.

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on the gmat, 'unlike' is used in essentially the same way as is 'like'. notice the problem that arises with prepositional phrases: there's no simple formal substitute for commonly spoken expressions such as Unlike in previous years, ... (which is considered unacceptable in formal usage because prepositional phrases aren't nouns). such expressions usually demand a complete overhaul of the wording of the sentence.

ajaypatil_am wrote:...at any cost

don't let the financial aid people see that.