An Interesting Note on the Spanish Imperfect Subjunctive
Recently I was reading a book in (Castilian) Spanish and I came across the word "estuviese" in the context where "estuviera" would be used.
Even in my Spanish classes, I had always learned the imperfect subjunctive was formed by adding "ra/ras/ran/ramos," etc, to the end of the third person plural preterit. Take "ellos fueron," remove the "ron," and add on that other "ra" ending.
As it turns out, there is another version of the Spanish imperfect subjunctive, which instead ends with "se/ses/sen/semos." So for the verb "tener," you could have:
Yo tuviera / Yo tuviese
Tú tuvieras/ Tú tuvieses
Él tuviera/ Él tuviese
Nosotros tuviéramos/ Nosotros tuviésemos
Ellos tuvieran/ Ellos tuviesen
Today, the form taught by Duolingo is the most common, but this other construction is still used some places, I believe most predominantly in certain regions of Spain. (The author of the book I was reading was from Barcelona; perhaps the construction in Catalan is similar? Or maybe those further from the tip of the peninsula favor this construction? I'd be interested if anyone here knows anything more.)
There are some subtle differences in use, but they are interchangeable in most cases. If you are trying to do a polite request--"quisiera un vaso de agua"--you shouldn't use "quisiese" in its place.
I don't believe Duolingo currently accepts this alternate form, but if you come across it in the wild, you'll know what it is!