I'm not a native speaker but a fellow learner of one year. With that being said, I found a Wikihow article that talks about the different ways to say "please" in Swedish (http://www.wikihow.com/Say-Please-in-Swedish).
Supposedly, "tack" is often used in informal settings for "please", such as in ordering drinks. An example given is "en kaffe, tack" ("A coffee, please"). This seems really similar to this Duolingo sentence of "ja, tack".
I'm guessing though that part of your confusion might have come from "snälla" being marked also as "please". According to that Wikihow article, it's supposedly used for begging and for telling someone to do something instead of actually asking. So to me it sounds like a kind of "please" that I wouldn't feel comfortable using that often.
But I don't know how accurate this stuff is. Can any native speakers out there confirm or reject it? I want to be helpful but I don't want to unintentionally mislead others either.
As a native speaker I would rarely say "Ja, snälla". It could however work well as an answer to a question that involves some effort from the questioner. "Do you want me to go out in the rain and dig up some potatoes for you?" - "Ja, snälla" (It is actually short for "snälla du" meaning "kind you".)
If you watch Swedish TV series or Swedish movies, you will only hear snälla to mean please when there's urgency in the statement, when someone is requesting something intensely. It's not typically used when someone asks if you want another beer and you say Yes, please. I
Yes, please (polite) and "Yes, you are so wonderful that I am sure you will do it for me" (flattering the other person to get your way). "Ja, snälla" can also be interpreted as "Yes, you would be nice if you did that". "Snälla" can be used when wanting to convince the person to do something they wouldn't really want to do (making them feel bad if they don't give you what you want). So my feeling is that "snälla (du)" (you kind person) as "please" can used to soften a request, kind of thanking in advance expressing how nice the person is known to be, or to express irritation for not getting what you want at once, all depending on context, tone of voice and so on.
Yes, that way of expressing yes is never written. I think it can be perceived as a way of pronouncing "jo" by inhaling. "Jo" is a special way of saying yes. In standard language I think that "jo" is only used in response to a negative statement, like: "- Du spelar ju inte gitarr." "- Jo, det gör jag." (- You don't play guitarr (as is known). - Yes, I do.)
I am a native speaker, using Swedish on Duolingo to learn the grammar to be able to somewhat explain the language to non-native speakers.
It is the same word. "Ja, snälla" is very informal, a short for "Ja, snälla du", which is also informal. The litteral meaning is "Yes, kind (person)", "Yes, you kind (person)". I think it is a common way to say it among some people, especially among female friends. More formal with snäll is, as has been pointed out in this discussion, "är du snäll" (litterally "är you kind" but used more as a phrase/statement than a question, so as "would you be kind to") like in "Räck mig saltet, är du snäll." (Hand me the salt, please) But that would be a very kind way of saying it. My sense is that the normal way is just state the desired in as few words as possible, for example seeking the attention at someone close to the salt and saying to that person "Jag skulle behöva lite salt." or "Kan du räcka mig saltet?" (I would need some salt or Can you hand me the salt?; that second sentence is quite polite in Swedish.)