So I wrote "Peter sometimes works", and Duo complained that I have an extra space between 'sometimes' and 'works'. It said the right answer is "Peter sometimesworks". I tried to report it, but the options I have are not relevant: o The audio does not sound correct. o The dictionary hints on hover are wrong or missing. o The Hungarian sentence is unnatural or has an error. So I can't report because none of the options fit.
In order to tell someone that Peter works sometimes, a native speaker of English will say something like, "Peter works sometimes." He will NOT say, "Peter is sometimes working."
In other words, the progressive tense (-ing form) would not be used. That is what I meant by my remark. Although the -ing form is used a lot in English, there are times when it is not used.
Your "counterexample" is indeed grammatical, but it is a different situation from the one that DL gives us. (Your example includes a "when" clause, which opens the door for the use of an -ing form.)