Learn Turkish: Telefonun pili yok. - The phone has no battery.

This sentence uses a definite noun compound because we are talking about a specific phone's battery. To show this relationship, telefon gets the genitive suffix -un (meaning "the phone's"), and pil (battery) gets the possessive suffix -i (meaning "its").

Why does the first word take the -un suffix, but the second word takes -i?

Both suffixes follow 4-way vowel harmony, but they look at the vowels in their own root words. Telefon ends with the rounded vowel 'o', so it takes -un. Pil contains the unrounded front vowel 'i', so it takes the matching -i suffix.

Why is it pili and not pilisi?

The 3rd-person possessive suffix is just a single vowel (-i, -ı, -u, or -ü). The buffer letter s (like in arabası) is only added when the root word already ends in a vowel. Since pil ends in a consonant, it simply takes -i.

How does yok translate to "has no" here?

Turkish doesn't have a direct verb for "to have." Instead, you state that someone's possession exists (var) or does not exist (yok). Literally, this sentence says "The phone's battery does not exist," which is the standard Turkish way of saying "The phone has no battery."