"Cây tre và đom đóm"
Translation:The bamboo tree and the firefly
Well, you can keep or omit the classifier "con" in this case. You can even omit "cây" before "tre" and turn the phrase into "Tre và đom đóm". However, this is not recommended because there's a reason for the definite article "the" to be in the phrase. :)
Is there a separate Vietnamese word for "bamboo" only? Or is it still "tre"?
There are actually more than one word for what translates to bamboo. Tre and trúc refers to bamboo trees/plants. Măng, though technically could be referring to any young plant shoot, almost always refer to bamboo shoots unless clarified. I'm sure there are more I can't remember.
Why can this be plural and singular at the same time? (bamboo trees or the bamboo tree)
The way Duolingo is teaching it, you need "những" or "các" in front to indicate plurality. The way the words are presented here, they're singular.
Are you sure you typed the right phrase (with correct diacritics)? See my result by Google Translate https://translate.google.com/#auto/en/d%E1%BB%85%20th%E1%BA%ADt%20%C4%91%E1%BA%A5y
so bamboo plant is also correct ̣even though I got marked wrong for it. In English we would not describe bamboo is being a tree.
A "bamboo rod" is a rod made from bamboo (cây gậy/roi bằng tre), not the plant/tree. Therefore, "cây tre" shouldn't be translated as "bamboo rod" but "bamboo (tree/plant)".
It is inconsistent then. There are questions that don't accept bamboo tree (and it corrected the answer to bamboo rod).