check for valid username and password
check for invalid username and password
type username and signin
type password and signin
leave everything blank and sign in

Also we have to use Black box testing techniques like BVA
and Equivalance Partitioning which are very important during
hese kind of situations.

We can split the scenarios for yahoo login as follows:

      UserName    Pwd    Actual

Sc1      I         I      Fail
Sc2      I         V      Fail
Sc3      V         I      Fail
Sc4      V         V      Pass

V-Valid    I-Invalid

1.Check if the login page enters correctly on giving the 
correct URL.
2.Check for the login page on giving the wrong URL.
3.Check that all the mentioned fields(i.e labels,text boxes 
and buttons) are present.
4.Check for login using the following details:
     a)Blank username and password
     b)Valid username and invalid password
     c)Invalid username and valid password
     d)Valid username and valid password
     e)Invalid Username and password
5.Check that the Login Button is functioning correctly.

Steps to follow:
   1) Check the visibility of fields, text boxes, button on
page.
    2) Click on Log in button without entering any value to
fields.
     3) enter Valid User name and Password.
     4) Enter valid User name and Invalid pwd.
     5) Enter invalid user name and valid pwd.
     6) enter invalid user name and invalid pwd.
     7) check log in button is working properly or not.
      8) And also check the functionality of forgot User
name and Password and Sign up.

check for entering spaces, daa with leading and trailing 
spaces
copy paste the password
check wheter password is encrypted
check for alert messages for invalid operations.

also check for the case sensitivity