It is based on the speaker. If you are a man, you say obrigadO, if a woman, obrigadA. It is the short for the meaning "i feel myself OBLIGATED to do you a favor..."., so the verb OBLIGATE (participle) is conjugated according to masculine or feminine of the person who is saying that...
The words "obliged" and "obligee" in English, are boroowed to the French "obligé", so it's also from a romance language.
(I'm a native speaker) -A man always use "obrigadO". -A woman can use both, but it's most common she use "obrigadA". -Talking about a group with both gender is better to use the male form (obrigado),(But women also use obrigada!) -This last rule is the same for all the references about groups of people with men and women inside it).
Example: A girl to a boy: Obrigada pela sua ajuda! (Thanks for your help!)
A boy to a girl: Obrigado pela sua ajuda! (Thanks for your help!)
A girl to a a group with both gender: Obrigada pela ajuda! or Obrigado pela ajuda! (Thanks for the help!)
I had a basic of Portuguese already before starting with duolingo, therefore I knew the difference between obrigada and obrigado. But I haven't heard an audio who says that right. Every single time there is obrigadA there is the MALE voice, every time there is obrigadO there is the FEMALE voice. It might seem nothing to who knows it, but it can be so confusing for who is first learning the language! I wanted to report it, but you can only choose two options (audio wrong and sentence not natural). Does anybody know if after choosing the report option you can add the specific error?
One might also say "yes, thank you" as a way of showing appreciation for concern being shown. For example, person 1 asks person 2: "are you feeling better now?" and an appropriate response would be: Person 2: "yes, thank you". (yes, thank you for your concern.) "Yes, please" wouldn't make sense in that case, so whether or not the two sentences are interchangeable would depend on the situation.