How to create dependent attributes in magento

Let me assume you mean product attributes by saying attributes plainly.

To answer your question its not possible to create dependent product attributes directly or straight forwardly under Manage Attributes. But you can make your attributes depend on each other by creating Configurable products.

Lets say you have 2 colors (black and blue) and 2 sizes (L,XL) and you want to make your colors depend on sizes.

1. At first create 2 attributes size and color of type dropdown under Manage attributes and create the attribute options and values. and Assign it to the right attribute set.

2. Now create the simple products , lets say product A with size L and color Black and product B with color Blue and size XL.

3. Now create a configurable product lets say AB and associate these two simple products A and B. Save the product.

if the customer opens the product AB and if they choose size L they can find only color Black. This is how you can make your product attributes dependent on each other.