Learn Turkish: Geyiğin rahat hissetmesi için zararsız bir çip seçtik. - We chose a harmless chip for the deer to feel comfortable.

This is how Turkish forms different-subject purpose clauses. To say 'for the deer to feel...', you literally say 'for the deer's feeling...'. The subject of the action takes the genitive suffix (making geyik into geyiğin), and the verb takes the verbal noun suffix -me plus the possessive suffix -si, all followed by için.

Why does the word geyik change to geyiğin with a ğ?

This is a classic example of consonant mutation. When the vowel-starting genitive suffix -in is added to geyik (deer), the hard k at the end of the word softens into a ğ.

Why are there two 's' letters in hissetmesi?

Hissetmek (to feel) is a compound verb created by combining the noun his (feeling) with the helper verb etmek (to do). When these two words merge into one, the final consonant of his doubles.

Why is the word order zararsız bir çip instead of bir zararsız çip?

In Turkish, when a noun is modified by both an adjective and the indefinite article bir (a/an), the adjective almost always comes first. The standard order is Adjective + bir + Noun.