In English, I am cold and I feel cold mean the same. In Esperanto, can we use "malvarma" for a feeling of the subject, or we have to say something like Mi sentas malvarmon? (In order to avoid confusion with My internal temperature is low.)
3 Answers
It is not erroneous. Mi estas malvarma and Mi malvarmas are acceptable translations. (You will find that the sentence Ĉu vi estas varmaj? appears in Engholm's Infanoj en Torento.)
However, some people dislike it and consider it ambiguous: it might mean "I am a cold person" or "I am hypothermic" or something like that. (In particular, Ich bin kalt/heiss is inappropriate in German, and so native German-speakers are most likely to be bothered by it.) Therefore, alternative translations are:
Mi sentas min malvarma. I feel [myself to be] cold.
Mi sentas malvarme. I feel cold[ly].
Al mi estas malvarme. Al mi malvarmas. It is cold for me.
"Mi estas malvarma" would describe a property of you, that you are a cold person.
I recommend using the verb "frosti":
Mi frostas.
For the opposite you can certainly say:
Mi malfrostas.
although in my experience
Estas al mi varme.
is more common.
I've never received any complaints when I said it that way, but you bring up a good point about ambiguity. I'm almost certain that saying "Mi estas malvarma." isn't an error, but using "sentas" instead of "estas" is probably for the best, if only for clarity's sake.