frequent error: "a" versus "an" in English
I am new to this site. I love it! But there seem to be some systematic errors. One is that it doesn't know when to use "a" versus "an" in English. For example it claimed that I made a typo when I wrote "An assignment" and said that I should have written "A assignment". It should be trivial to fix this. "A" comes before a consonant and "An" comes before a vowel. I can't think of any exception to this rule.
The appropriate use of 'a' and 'an' is but one of a number of errors that crop up with some frequency. It's sort of annoying to provide an answer in good english and be corrected with something that's incorrect. I suppose the only thing we as a community can contribute is to report these errors so they can be corrected by duoLingo. Errors are easily reported on the web version of duoLingo -- more troublesome for iPad / iPhone users.