Learn Turkish: Hipotermi geçirecekler diye sıcak çorba hazırladım. - I prepared hot soup for fear that they will get hypothermia.

You likely already know diye as 'because' or 'saying'. When paired with a future tense verb like geçirecekler (they will get), it expresses the thought or worry that motivated the action. Literally, the sentence is structured as: thinking 'they will get hypothermia', I prepared hot soup. This is the standard way to express 'in case' or 'for fear that' in Turkish.

Why use the future tense geçirecekler when the main verb hazırladım is in the past tense?

Turkish doesn't shift tenses to match the main verb like English does. Because diye acts as a quote for your internal thoughts, it captures the exact thought you had at the time. When you were making the soup, your thought was, 'They will get hypothermia' (Hipotermi geçirecekler). You just report that original thought directly.

Why is the verb geçirmek used for 'getting' hypothermia?

While geçirmek usually means 'to pass' or 'to spend (time)', it is the standard verb for suffering or undergoing a sudden medical condition, trauma, or illness. You will also see it in common phrases like kalp krizi geçirmek (to have a heart attack) or kaza geçirmek (to have an accident).

Could I use için instead of diye to say 'because' here?