Translation:He wanted to win again, but he lost.
I am a little confused about the verb aspect in this sentence. To me, it would seem that "he" is interested in a single act of wining again which, as a single act, would seem to require the perfective "vyhrát": "Chtěl zase vyhrát, ale prohrál". This even more so as the perfective form "prohrál" is used, supposedly indicating that we are talking about a single instance.
If I had to guess, I'd say that the use of the imperfective form here has to do with "zase"... but it still isn't entirely clear to me why...
Right, but if we did want to suggest repeated winning, we'd be more likely to say "He wanted to keep winning" or "He wanted to win more" rather than "He wanted to win again." I suppose I can see how a charitable interpretation could understand "He wanted to win again" as talking about repeated winning, but it doesn't seem to be the most natural interpretation to me... I don't know...
It is mostly a byproduct of my formal instruction in Russian (it too has aspect) mixed with my semi-native familiarity with Czech. That being said, this website gives a quick but good overview of aspect use in Czech. http://www.locallingo.com/czech/grammar/verbal_aspect.html
Nueby also posted some good resources a while ago (which probably ought to be stickied in the Czech discussion board if possible). This one covers aspect starting on page 47: http://www.factumcz.cz/K.Tahal-Grammar.pdf And this one starts covering aspect on page 140: http://utkl.ff.cuni.cz/~rosen/public/GGG/Czech_essent_grammar.pdf