How do you write a test cases for registration form of any website?

IT's very easy to write test cases for normal registration form. This may be functional or UI dependent test cases. for UI ,we just check the requried elements are present or not... Where as in functional,We have to check the functionality also,i.e; for example ,if it a web edit box taking u r name then we have to check Wheather it is taking correlty or not, which is mentioned by the developer...... here generally rise one Question.. how we know about all those things. So,for this reasion only we have to understand total flow of application..... first it seems to be difficult, But once u understand the flow ,u can......... IT's very easy to write test cases for normal registration form. This may be functional or UI dependent test cases. for UI ,we just check the requried elements are present or not... Where as in functional,We have to check the functionality also,i.e; for example ,if it a web edit box taking u r name then we have to check Wheather it is taking correlty or not, which is mentioned by the developer...... here generally rise one Question.. how we know about all those things. So,for this reasion only we have to understand total flow of application..... first it seems to be difficult, But once u understand the flow ,u can......... IT's very easy to write test cases for normal registration form. This may be functional or UI dependent test cases. for UI ,we just check the requried elements are present or not... Where as in functional,We have to check the functionality also,i.e; for example ,if it a web edit box taking u r name then we have to check Wheather it is taking correlty or not, which is mentioned by the developer...... here generally rise one Question.. how we know about all those things. So,for this reasion only we have to understand total flow of application..... first it seems to be difficult, But once u understand the flow ,u can.........