"This is not a radio, mom."
Translation:Это не радио, мама.
Dash is for "is", and coma is for situation when you want to address someone. Just like in English.
Мама - это не радио. - Mom is not a radio :)
First sentence with Dima doesn't make sense. Dash should be replaced with a comma, because you're saying to your mom that Dima is a medic.
if you wish to learn Russian, surely you want to write it as well - not only read it ?
If you are on a computer, you must install the Russian language and can use 'on-screen keyboard' found within the accessories->accessibility menu to enter cryllic alphabet . you will need to switch between EN and RU to complete the duolingo lessons. I also recommend having a notebook for RU lessons, and take some time to write the words and letters down. there is some success in retaining these lessons when you write them, and have to re-read them back from your own writing.
it sounds complicated but takes some tries and practice, do not be discouraged! We are all learning Russian together and will face many challenges
More info for windows-pc: https://support.microsoft.com/en-us/help/14236/language-packs
More info for mac: http://learnrussian.rt.com/speak-russian/how-install-cyrillic-keyboard-mac
if you need more help please reply, i will do my best to advise
It would be useful to have audio, and perhaps additionally 'phonetic' spelling. This being a very early beginner stage. I laboriously (this time) facilitate my desidirata by copying the Russian translation from this page, to Google Translate app, starred for my 'Lexique'. There I can play the audio and better recognise the cyrillic words. Of course, this one is dead easy.