Learn Turkish: Telefon yastığın altında. - The phone is under the pillow.

To say 'under the pillow' in Turkish, you literally say 'at the pillow's underside'. So, yastık needs the possessor ending -ın to mean 'the pillow's'. When adding an ending that starts with a vowel to a word ending in -k, the k softens into a ğ (soft g), making it yastığın.

Break down the word altında for me. Why isn't it just altda?

Spatial words like alt (bottom/under) work like possessed nouns in Turkish. altında is made of four parts: alt (bottom) + ı (its) + n (a buffer letter used before location endings) + da (at/on). Together, it literally means 'at its bottom'.

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

There isn't a separate word! In Turkish, the 'to be' verb (am/is/are) is usually left out for the third person ('he/she/it is'). You just state the subject and its location: Telefon yastığın altında (Phone pillow's at-its-underside).

How do we know it means 'the' phone and 'the' pillow if there is no word for 'the'?

Turkish doesn't have a direct word for 'the'. However, subjects at the beginning of a sentence (Telefon) and words that have a possessor ending (yastığın) are naturally understood as specific, definite things by default.