They are not always synonyms. Just as in English, Hindi has two different words for "Talk" and "Speak." We would not say "I am talking English," right? "Speaking" would be the right word to use in that sentence. Perhaps either one would be gramatically correct in this sentence, but duo does want to teach you both verbs and their exact translation, because they will not always be synonymous, and the distinction will be useful to know for those times.
Thanks for the clarification! It is perfectly reasonable and I understand. Still, the behavior of the program in such instances as this does not let me know whether what I put is acceptable in this particular sentence, no matter that “speak” and “talk” and their Hindi equivalents do act differently in other examples. It is a tricky instance, I see that, and perhaps not the best one to use. Still, I take your point I take your point that the two verbs ARE different, and it is important to learn which is which. Thanks for your attention to this.
I guess the point of the lesson is to get you practice with using the verb to talk. The sentence might still be grammatically correct if you use a synonym for talk but that doesn't help you to get practice using the word talk and it would be ideal to be able to use several different words for one meaning
What I believe is that speaking is something an individual can do by themselves without there being anyone else there to listen. While talking or a conversation requires at least two people.
बात मत करो is usually used by teachers in schools when the kids are blabbering and they tend to point a particular kid out and tell them to stop speaking in order to refrain the others from talking.