Learn Turkish: Kutu dün doluydu. - The box was full yesterday.

The root word is dolu (full), which ends in a vowel. The past tense ending for 'it was' here is -du. In Turkish, when we attach this past tense ending to a noun or adjective that ends in a vowel, we must add the buffer letter -y- between them. So, dolu + y + du becomes doluydu (was full).

Why does the past tense suffix use a u (-du) instead of an ı (-dı)?

This is because of 4-way vowel harmony. The last vowel in the word dolu is u. According to the rules, if the last vowel is o or u, the suffix must also use u. This is why the ending is -du rather than -dı, -di, or -dü.

Could I put dün (yesterday) at the beginning of the sentence?

Yes! In Turkish, time words like dün often go at the very beginning of the sentence (Dün kutu doluydu) or right before the verb/adjective (Kutu dün doluydu). Both are perfectly correct, but placing it right before the adjective adds a little more emphasis to the time.

Where is the word for 'the' in this sentence?