Learn Farsi: من باید یک پیانویی را کوک کنم.

This is the circumpositional indefinite marker, meaning it wraps around the noun. Using both yek before the noun and -i after it emphasizes that you are talking about a certain or a particular piano, rather than just any random piano.

Why is the word pianoyi with a y sound, instead of just adding -i?

Because piano ends in a vowel sound (o), we need a bridge consonant to connect it smoothly to the -i suffix. In Farsi, the y sound is used to link them, making it pianoyi.

Why is the object marker ra used here if the piano is indefinite (a piano)?

This is a very important rule! The word ra marks a specific direct object, not just a definite one (the). Because you are talking about a certain piano (one specific piano you have in mind, even if the listener doesn't know which one), Farsi treats it as specific enough to need ra.

Why does the verb use the form konam instead of the regular present tense mikonam?

The word bayad (must) requires the following verb to be in the subjunctive mood, which is used for obligations and desires. For the compound verb kook kardan (to tune), you drop the mi- prefix from mikonam to get the subjunctive form konam.