"Ленив" answers the question "каков". "Ленивый" answers the question "какой". "Каков" questions the quality of something/one. And the respective forms give us information about the quality of something. for example: красен дом = the house is of the red color (or beautiful for red is a synonym of beauty in russian). высок дом=the house is a tall one. "Какой" questions either quality or an attribute of something. In other words какой is more about identifying an object by some parameter. Sometimes it can be used instead of каков, but it doesn't carry the equal emphasis on the _quality_ of a described object, more inclining just to specify the object more precisely. красный дом = a red house. высокий дом= a tall house. Хороша погода= the weather is Great now! Хорошая погода=a good weather. easy :). ps. regading "голубо": "а голубо сегодня небо!" - sounds fine, though a bit old stylish. but it's something i could say when i am walking taking pictures. Прозрачна вода в реке. And so on.

I don't see any difference in meaning between these two forms. :) But I prefer the short one. Небо было серо, город был молчалив. Небо было ясно, город был полон звуков. But Небо было голубым. I'm not sure why, but I would never say Небо было голубо. It sounds strange to me. So I think It depends on the adjective.

Basically, the short-form adjectives imply the current condition, while the long adjectives -- the quality in general. For example: Я болен -- I’m sick (eg I’ve got a cold) Я больной -- I'm unhealthy But very often they are interchangable, so it depends on the particular adjective. Он стал ленивым / Он стал ленив. -- the same. Usually, the short adjectives are more common in formal writing. With 'это' you should use the short form: Это будет полезно; Это будет хорошо etc.