I have a question about the use of instrumental case and the past tense with the verb быть. I know that when you want to say something like "When I was a boy", you would use instrumental case like so: Когда я был мальчиком, and I know that to say something like "I had a dog" does not require instrumental case: У меня была собака. However, I'm still not entirely sure exactly when to use the instrumental case with быть, such as in the case of "She was a smart girl". I've seen both cases occur as "Она была умная девушка" and "Она была умной девушкой". What are the rules regarding this? It almost seems to me, that it could be a question of whether the situation has changed. If the girl was smart, and still is, then instrumental is not needed, but as in the case of "When I was a boy", I'm currently no longer a boy, so I would need to use instrumental in this instance. Am I correct?
My second question: where can I find a Russian dictionary site that explains the meanings of Russian words in Russian, kind of like a Russian version of "dictionary.com"? I've been reading a Russian book, and I make virtual flashcards of all the words I don't know, and I use Google Translate and Wordreference to help, but I know that these sites are not always accurate, because in many cases direct translations between languages don't exist. I think that learning new vocabulary through a dictionary that's not a "Russian-English" dictionary would be great supplemental help.
Lastly, I have a question about причастия. When I studied in Russia, we covered a lot of Russian grammar, including причастия, and I now feel that I have at least a fairly decent grasp of grammar. However, while I can understand причастия when I encounter them in Russian text, I find it very difficult to use them correctly myself. My question is, how often do причастия appear in actual conversation? Am I correct in presuming that they are more or less only used for literary purposes? This would decrease my frustration with them.