Learn Hardcore Turkish: Sıcak yaz günlerinde serinletici bir duş almak harika. - Taking a refreshing shower on hot summer days is wonderful.

It’s actually three pieces glued together:
1) -ler = plural (“days”)
2) -in = 3rd-person possessive (“of the days”)
3) -de = locative case (“in/at”)
So gün-ler-in-de literally means “in the days”, i.e. “in those (hot summer) days.” Plural nouns need that possessive suffix before you add a case ending.

Why is serinletici used instead of serin?

serin simply means “cool.” serinletici comes from the verb serinletmek (“to cool/refresh”) plus the adjectival suffix -ici, so it means “refreshing” (literally “that which causes coolness”).
– serin bir duş = “a cool shower”
– serinletici bir duş = “a refreshing shower”

Why is there a bir before duş, and why doesn’t duş take an accusative -u?

Turkish doesn’t require an indefinite article, but bir corresponds to English a/an. When you use bir with a singular noun as an indefinite object, you do not add the accusative suffix.
– Correct: serinletici bir duş almak (“to take a refreshing shower”)
– Incorrect: serinletici bir duşu almak

Why is almak used here instead of duş yapmak?

The idiomatic Turkish for “to take a shower” is duş almak (“to take” + “shower”). While duş yapmak exists, speakers overwhelmingly prefer duş almak in everyday usage.

What role does harika play, and why is it at the end of the sentence?

harika is an adjective meaning “wonderful” that functions as the predicate describing the action (“… taking a refreshing shower is wonderful”). In Turkish, predicates (especially adjectival or nominal ones) often come at the end, and the copular verb olmak is dropped in casual contexts.

Could we add the copula -dir to harika?

Yes. For a more formal or assertive statement, you can say harikadır. In everyday speech, dropping -dir is standard, so harika alone is perfectly normal.

Is the word order in this sentence fixed? What if I wanted to move sıcak yaz günlerinde elsewhere?

Thanks to Turkish’s case system, word order is relatively flexible. The most natural order is:
1) Time/place phrase (sıcak yaz günlerinde)
2) Object phrase (serinletici bir duş almak)
3) Predicate (harika)
You can reshuffle for emphasis or style—e.g. “Serinletici bir duş almak, sıcak yaz günlerinde harika”—but the original arrangement is the default in spoken and written Turkish.