Turkish endings change their vowel to match the word they attach to. Two-way harmony picks between e and a; four-way harmony picks between i, ı, u and ü. Everything you add - plurals, cases, question particles, person endings - obeys it, so it is the rule underneath every other rule.
Examples
- Evde kimse yok.
- There's nobody at home.
- Okulda çalışıyorum.
- I'm working at school.
- Kitapta bir resim var.
- There's a picture in the book.
The mistake to avoid
The same ending has two or four written forms - -de and -da are one ending, not two.
Where this fits in
Greetings & introductions, Turkish A1: Say hello, give your name, ask someone else's, and where they're from.