...a verb, a subject noun and an object noun!

A verb is a word in a sentence that expresses an action, an occurrence, or a state of being.

A noun is a word that is the name of something and is typically used in a sentence as subject or object of a verb.

A subject noun is a noun that performs the action of the verb in the sentence.
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An object noun is a noun that receives the action of the verb.

A few years back I learned this technique from a great Business Analyst I worked very closely with. It was very early in my career as a tester at that time. She taught me that whenever we gather, analyze, interpret or elaborate business requirements of a software applications, we should always start with these three words (or phrases): a verb, a subject noun and an object noun, to identify all the business functions that the application is supposed to perform. Once we have the list of all the functions documented in this form, we should then elaborate on the individual functions to come up with the details of the requirements and/or test cases.

Since then, I has been tremendously helpful for me in writing good quality test cases!

Let's take the example of the deposit function in an ATM machine. 

Here, the verb is 'deposit'.

Now, who deposits and what is deposited?

Clearly, an account holder deposits.

But which account holders? Checking Account? Credit Cards? Mortgage? Domestic? International? 

Here comes the need for elaboration. We should document very explicitly what types of account holders can use this function.

Similarly, what is being deposited? The answer is simple. Money.

But, in which form? Cash? Check? This again needs elaboration.

This process can be extended to identify all business functions of the ATM machine.

Note that, this technique is not specific to any methodology and can be used in Agile, Waterfall, Iterative or anything similar.

Here are a few examples:

Account holder withdraws money
Account holder views balance
Guest opens account
Administrator prints report, etc.

Over the years, this technique has been of great help to me as a tester to identify all positive and negative test conditions.