Translation:Foreign exchange students came on Thursday and Friday.
the reason because this sentence uses が instead of something more simple is because the thing you want to make emphasis in this sentence is that it was "the exchange students" that came.
So lets say we want to say it in a more basic way, the way duolingo has taught us so far, you would try:
"留学生は 木曜日と金曜日に 来ました"
but in japanese there is a rule where you usually put "temporal nouns" as "friday", "tomorrow" and "right now" first in any sentence.
So here is peculiar phrasing you could also try so you don't break that rule:
"木曜日と金曜日に 留学生は 来ました",
but then you are just declaring the students as the topic and not as the thing that is important here (aka the subject), the topic is not the subject in this case, the topic is that "it was thursday and friday when they came" so you might try:
"木曜日と金曜日は 留学生は 来ました"
but now you have 2 topics and not explanation on what happened to those 2 things in relation with each other.
The reason why you use が here instead of は it's because its usually associated that when you use が everything to the left of where you put it has more importance, and when you use は everything that put over the right of the particle has more importance in relation to what you indicate as the topic. This is not really a rule but a good way to understand the difference between は and が in certain cases, so when you say:
"木曜日と金曜日に 留学生が 来ました",
you are letting the other person know that it was exchange students that came those day, that's the important part.
Because you're saying "on Thursday and Friday", rather than "about Thursday and Friday: …". Although in fairness "Regarding Treeday and Goldday: (some) foreign exchange student(s) came to the school" might be a reasonable way to write about it in your diary 『木曜と金曜は学校に留学生きました』. Or maybe that should be 学校へ, I'm not too sure with this yet.
I hate this question only because of the fact that the audio version of it is such a jerk on this. From what I gather, りゅうがく生 is expected. Even though 学 was taught ages ago and there's no reason 留学生 should be rejected. Its also fun always having to go back and replace the 曜 in the middle of every day with よう because I don't know what Duo will accept there and its just not worth risking it.
Punishing the desire to try different (correct) ways to say the same thing is the opposite of what a language teaching site should be doing. Now its only exacerbated with the bonus points/leaderboard combo. At the very least they need to go through and fix these situations where the correct Kanji is not accepted in the audio-only versions. It has been months.
Haha, yes. I have that difference between my computer, tablet and phone too! I guess that's because the latest Japanese tree updates roll through more and more of the userbase as it is considered stable by the software developers at Duolingo. So, there's nothing you can do about it, just wait a while and they'll make the new tree the standard tree. :)
Yes, a very good one: the students are the subject of the sentence, not the object. As a very general rule of thumb, those who "do" the action (as described by the verb) get が and those who "undergo" the action get を.
Saying 留学生を来ます would be like "I come the exchange students".
dear duolingo, when typing on my computer with a japanese keyboard, りゅうがく生 takes way, Way, WAY longer than 留学生. can you please just let us use kanji? And while we're at it, you'll accept 木曜日 for Thursday in other places but won't accept 金曜日 for friday. Only 金よう日 works. Seriously. I want to use my keyboard so I can't just blunder my way through word banks, but you aren't making it easy.