Learn Hardcore Turkish: Seçimde oy vereceğim.

It’s the locative case, meaning “in/at/on.” So seçimde = “in/at the election.” Turkish uses the locative suffix -DA with four forms: -da, -de, -ta, -te. Which one you use depends on vowel harmony (a/e) and consonant voicing (d/t).

Why specifically seçim-de (with -de) and not seçim-da or seçim-te?

The last vowel of seçim is front (i), so you choose the front version -de (not -da).
The stem ends in a voiced consonant (m), so you use d (not t). The -t- version appears after voiceless consonants (mnemonic: fıstıkçı şahap: f, s, t, k, ç, ş, h, p).

Where is the subject “I”? Why isn’t ben used?

It’s encoded in the verb ending. In vereceğim, the final -im (spelled as part of the fused form) marks 1st person singular. Turkish usually drops subject pronouns unless you want emphasis or contrast. You can say Ben seçimde oy vereceğim to stress “I.”

How is vereceğim formed?

Root: ver- “give.”
Future: -ecek/-acak → verecek-.
1st person singular: -im → underlyingly verecek-im.
Phonetic change: the k between vowels becomes ğ → vereceğim. So: ver + ecek + im → vereceğim.