"The fish bites the mug."
Translation:Con cá cắn cái ca.
No, it does not. Due to the difference in tones between <cá c-...> (rising) and, sorry, <cặ c-...> (deep down), no misunderstanding would be made saying or hearing that part. Vietnamese writings without tone marks, on the other hand, may lead to misunderstanding as in your example "con cac can cai ca" -> "the cock/penis bites the mug" -> It's a monster! Run for your life!
The Vietnamese for Windows works fine for me: cắn. which is the a key then the 1 key not shifted and then the 8 key also not shifted. I tried zooming in but yours looked just like mine. However, if you use a different accent from a different language keyboard it won't recognize it even if it looks similar. http://typingvietnamese.blogspot.com/p/typing-vietnamese-for-windows.html
Sometimes I use the MS OS keyboard and when the sentences in Vietnamese are correct and exactly as the model answer, Duo shows the message "Almost correct!" (and when I begin the course many sentences were rejected and now they are accepted but not as fully correct). Then, using TELEX the sentence is fully correct.
I think I am not the only one with this problem. Anyways, I like to alternate the use of both systems. ;)
UniKey is what all of Vietnam uses so it's probably best to use that. It's an unintrusive little program that works very nicely. To type cắn, I just type cawns. For learners, this typing method also helps separate the tones from other diacritics, and reinforce the idea that the tone is a property of the whole word, and not just of that one vowel on which it happens to be written.