>>The way we learned to count syllables was "count the vowels". Or, if you're not sure, try to sing the word, one syllable - one note. Including the funny cases where the song requires the singer to extend the r-as-vowel (which actually happens quite often - drvo (tree), srce (heart), prvi (first), rđa (rust)... and, surprise surprise, Hrvatska (Croatia) and Srbija (Serbia) :).
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>You wouldn't have to if you didn't loose vowels in those words in the first place. :)

Actually we share the blame with Slovenian, Croatian, Macedonian and Bosnian. After so many centuries, it makes no sense looking for these vowels... they're just gone. And I have no hopes in archaeology either, they won't find them.

>In Russian: derevo, serdce, perviy, ruda.

I know... you've inserted a few vowels because it was too hard to pronounce them the proper way :). Though one would think you would benefit from that, living so much more to the north - pronouncing r as a vowel gives you more friction, and therefore warms you up much better. Saying "brrrrr" in English or French never gets you far... too smooth.