'the' is a very difficult subject. Articles in English grammar such as 'the' many English language learners will never get right.
So if we are talking about something we would often use 'a' or 'an' = 'sebuah', 'seorang' etc. if singular, eg. 'he ate a cake' = 'dia memakan sebuah kue' or no article if plural eg. 'he ate cakes' = 'dia memakan kue-kue'. However if we know what the subject is we would use 'the'='ini', 'itu', '...nya' so for instance I may have just told you I bought or made cakes eg. he ate the cakes = 'dia memakan kue ini/itu' or 'dia memakan kue'.
'the' would be used when referring to something generalised eg. 'I play the guitar (any guitar)' = 'saya bermain gitar'. 'this' = 'ini' and 'that' = 'itu' are used when referring to specific object(s) so 'I will play this guitar' = 'saya akan memainkan gitar ini'. However you dont need to use it when talking about things in general eg. 'I play guitar' = ' saya bermain gitar'.
However there are many exceptions to usage in particular cases and other ways 'the' is used as well (eg. uncountable nouns)!
In this case 'winter is strawberry season' should also be correct, as you are talking about winters and strawberry seasons generally not just one season. If marked as wrong raise it with duolingo.
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