Can we have a little clarification about this? If "tymor" also means "season," I'm not sure why "tymor y haf" doesn't mean "summer." It's especially puzzling for me as an American, because we don't really have named school terms, so I'm not sure what the equivalent would be -- summer session, maybe, but only at university.
It's a bit difficult to translate out of context. If you're talking about schools, colleges and universities in Wales, it translate as "summer term". But if you were involved in the tourist trade, you might be referring to "summer season" when there are lots of holidaymakers around.
Just to add; the school/college/univeristy terms in the UK are usually referred to as 'Autumn Term', 'Spring Term' and 'Summer Term' - in case a similar question appears. The period between Summer Term and Autumn Term is known as '(the) Summer Holiday(s)' or occasionally 'Summer Break'.
Right, the syntax is the same: possessed (+ article) + possessor. The difference is that in Irish there is an actual genitive case so you have to change the form of the article and noun (+ adjective, if applicable). In Welsh, cases were lost a long time ago, as a result you use the same syntax as Irish without all the hassle of declension.
'The summer season' is already one of the alternative answers. If it is refused again, make sure that you are using the latest version of the app - there are frequent updates. If your app is up-to-date, please raise a 'Duolingo bug report' so that Duo staff can investigate.
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Haf comes from Proto-Brythonic *haβ̃, a descendant of Proto-Celtic *samos, and is cognate with English summer, Armenian ամ (am, “year”) and ամառ (amaṙ, “summer”), Central Kurdish hawîn ("summer"), Khotanese hamana- (“summer”), regional Persian هامین (hamîn, "late summer") and Sanskrit समा (samā, “season, year”). It also appears in Welsh Gorffennaf ("July", literally "end of summer").