Questions & Answers about Kahvaltıda bir portakal yedim.
Because -da / -de / -ta / -te is the locative ending, which often means in, at, on.
- kahvaltı = breakfast
- kahvaltıda = at breakfast / during breakfast
The exact form changes because of vowel harmony and consonant voicing rules:
- kahvaltı ends in the vowel ı, so the suffix vowel becomes a
- the final consonant in the suffix is d here, giving -da
So kahvaltıda means at breakfast.
Bir can mean one, but it also often works like a/an in English.
So:
- bir portakal = an orange / one orange
In this sentence, it most naturally means an orange, not necessarily emphasizing the number. But depending on context, it could also stress one orange.