how to make a field mandatory for certain field values from a picklist / cascading Picklist

 Hello Community,

i'm working on a prestudy for a Customer, and i have two problems/questions:

1- i would like to make a Field as mandatory when the user choose a special value from a picklist, for example if the user choose value A from a picklist than he will have to write a small text in a text Fiel.

Can anybody tell me how this can Work?

2- i have two Fields, Sales Stage and Reason Won/Lost, i would like to create a cascading Picklist between this two Fields, the problem is that, in the parent and related Picklist i can only see the Reason Won/Lost Picklist but not the Sales Stage, even it's by default and a requierd Field.