Learn Turkish: Soğuk havada hasta oldum. - I fell ill in the cold weather.

The suffix -da (or -de, depending on vowel harmony) means "in", "on", or "at". Here, it attaches to hava (weather) to make havada, meaning "in the weather".

I thought olmak means "to be". Why does hasta oldum mean "I fell ill"?

While olmak often translates to "to be", it also means "to become". In Turkish, it is very common to combine an adjective or noun with olmak to create a verb. Hasta olmak literally means "to become sick".

How do we know it was "I" who got sick if the word ben (I) isn't in the sentence?

In Turkish, the subject is built right into the verb. The -m at the very end of oldum specifically means "I did it". Because of this, you don't need to use the separate word ben unless you want to add extra emphasis.

Why does the sentence end with the verb oldum?

Turkish naturally uses a Subject-Object-Verb word order. This means the main action of the sentence almost always comes at the very end. The background details (soğuk havada) come first, followed by the action (hasta oldum).