For writing any test cases we need requirements.
Any way we are very familiar with Yahoo mail login window.
LoginWindow will contain following fields.. 

Element                               Type
-----------------------------------------------------------
Yahoo! ID                            -TextBox  
Password                             -TextBox 
Remember my ID on this computer      -Check box
Forget your ID or password?          -Text link
Login                                -Button 
------------------------------------------------------------
testcase format

TestacaseID TestDescription    Input       Expected Value

sorry here the text box is not enough to type the format of 
the test cases i am giving the test inputs both for 
positive and negative test cases based on this u can write 
easily test cases

Here to give or to generate valid and  invalid  i/p values 
We will use input domain testing and Boundary Value
Analysis(BVA) techniques.

For Eg.

Probable positive one is
Valid, Valid  set of values ? Are inputs for positive 
                              testcases 
Valid ,Invalid               - Remaining all are i/ps 
Invalid ,Valid                 for negative test cases
Invalid, Invalid 
Empty ,Valid
Valid ,Empty
Empty,Empty 

even we can check the functionaly of the text links
and check box functionality
help text link functionaly ....etc

Do always remember onething the number of testcases mustbe 
greater than or equal to number of fuctionalities on the 
screen.

this is because for every functionaliy there will be one 
definate positive testcase minimum.

some times it may have more,let us take the login window
for checking authentication of password and login id 
we will go for all possible permutations and combinations
i.e valid,valid,invalid,valid.....like shown above.

still you are unable to write and want to see the test cases
please mail me i will send you the test cases with analysis.

mail me at: rajendra_penumalli@yahoo.com
or if you have any doubt please call me 9885162742
(hyderabad)

1.successful opening of IE.
2.Blank page should be opened by default.
3.Enter invalid URL in the address box and check for the 
error message.
4.Enter blank spaces,special characters,numbers etc and 
check for the functionality.
5.Click the select option on tool bar for selecting the 
address of Yahoo.
6.For successful selection of Yahoo address it should open 
corresponding page.
7.Enter valid URL in the address box and corresponding 
Yahoo page should open.
8.After sucessful opening of Yahoo login window,the cursor 
should by default point to the Userid text box.
9.Successful opening of Yahoo login window due to the valid 
URL placed in address bar.