Learn Turkish: Bu akşam patates pişiriyorum. - I am cooking potatoes this evening.

In Turkish, when you talk about a general category of things as a direct object—like "cooking potatoes" or "reading books"—the noun usually stays singular. Saying patatesler would sound like you are referring to specific, individual potatoes.

Why doesn't patates have an accusative case ending here?

We only use the accusative case (adding a vowel suffix like -i) for specific direct objects, which usually translates to using "the" in English. Because you are just cooking potatoes in general (an indefinite object), patates stays in its bare, dictionary form.

Is Bu akşam an indefinite noun compound?

No. Even though it's two words, bu (this) is a demonstrative adjective describing the noun akşam (evening). An indefinite noun compound requires two nouns, where the second noun gets a possessive suffix—for example, akşam yemeği (evening meal / dinner).

How do you build the verb pişiriyorum?

It comes from the dictionary verb pişirmek (to cook). You drop the -mek to get the stem pişir. Then, you add the present continuous tense suffix -iyor (am/is/are doing), and finally the "I" personal suffix -um, giving you pişiriyorum (I am cooking).