The definite article could also be absent from the English translation, as the context isn't clear on whether we're talking about specific harmonicas or harmonicas in general. Both 'the harmonicas' and 'harmonicas' are correct in this situation.
Invatat = learn what is for play=? In romanian? And is the meaning in romanian sentence is that harmonicas are light (not heavu) or that they are easy to play with? For me second is closer but will be neter to be sure