I have discussed this issue with Jason, Papasmurf ,ブンフイ and others. http://www.italki.com/Answers/question/25398.htm Papasmurf said: "wakaru" is something like "is understood" (rather than "understand"). So saying "nihongo ga wakarimasu ka?" would be litterally like saying "is English understandable to you?" (although that sounds awkward). ブンフイ said: The particle が is used here because the verb involved is not an action verb. わかります (understand) does not involve any action. I think some of Japanese verbs(and adjectives) are just like reflexive verbs in Spanish. Me gusta sushi. Watashi wa sushi ga suki desu. In both cases, the real sublect is not 'watashi' but 'sushi'. That's why we can't use 'wo' for these words. So far I found only several words like that. 'wakaru', 'suki', 'kirai', 'yoi(good)' ex) watashi wa sushi ga yoi desu. (I'll order sushi...we say like this at restaurants)

Additional comment: "nihongo wo hanashimasu", you can also say "nihongo ga wakarimasu". So depending on the relation of the verb to the object, the conjunction varies. It's wrong to say "nihongo wo wakarimasu". I know, Japanese is tricky, unfortunately. I'm afraid I cannot give you when to use "ga" and "wo". Perhaps you can consult a book on grammar.

'ryouhoutomo nihongo ga wakarimasu ka' You need to use 'ga' when you want to make sentences using 'wakarimasu'. 'nihongo wo wakarimasu' is incorrect. As you said, 'ga' is used for emphasis on the object and taking up topics. The speaker wanted to know if they both understand 'Japanese' among other various languages. As for 'ryouhoutomo', it is a common word. Please look up '-tomo' in your dictionary. I hope this was helpful.