Learn Turkish: Yolculuk çok kötüydü, otobüste uyuyamadım. - The journey was very bad, I could not sleep on the bus.

It is built like this: uyu (sleep) + y (buffer letter) + ama (unable to) + dı (past tense) + m (I). Because the verb root uyu ends in a vowel and the inability suffix -ama starts with a vowel, we must put the buffer letter -y- between them.

Why does "kötüydü" (it was bad) have a "y" in the middle?

The word kötü (bad) ends in a vowel. When you attach a past tense ending directly to a noun or adjective that ends in a vowel, you must use the buffer letter -y- to connect them. So we get kötü + y + dü.

Why is the ending on "otobüste" (on the bus) spelled with a "t" instead of a "d"?

This is because of consonant mutation. The word otobüs ends in the hard consonant "s". When a suffix that normally starts with "d" attaches to a hard consonant, it changes to "t" to match the hard sound. So the location suffix -de (in/on/at) changes to -te.

Why does the past tense part of "uyuyamadım" use "dı" instead of "di" or "du"?

Vowel harmony! The past tense suffix always looks at the vowel right before it. Here, it attaches to the inability suffix -ama, which ends in "a". Following the rules of Turkish vowel harmony, an "a" is followed by an "ı", making the past suffix -dı.