Learn Farsi: تو باید یک پیمانه روغن بخری.

You would use the word چقدر (cheghadr), meaning "how much", right before the uncountable noun روغن (oil). The question would be: تو باید چقدر روغن بخری؟ (How much oil must you buy?). The structure of the sentence stays exactly the same; you just replace the specific quantity (یک پیمانه) with چقدر.

Why isn't there a connecting word like "of" between پیمانه (cup) and روغن (oil)?

In English, we say "a cup of oil", but in Farsi, when you use a measurement word (like cup, kilo, or glass) before a noun, you just put them side by side. You don't need a connecting word, and you don't even use the ezafe linking sound here. It's simply یک پیمانه روغن (one cup oil).

Why is the verb بخری (bekhari) instead of the normal present tense می‌خری (mikhari)?

This is because of the word باید (must). In Farsi, words like "must", "can", or "want" require the main verb to be in the subjunctive form. To make the subjunctive, we swap the continuous می (mi-) prefix for the ب (be-) prefix, which gives us بخری.

Why is بخری (buy) at the very end of the sentence, so far away from باید (must)?

Farsi naturally puts the main verb at the end of the sentence. The "helper" word باید (must) comes early, usually right after the subject. All the extra details—like the quantity and the object (یک پیمانه روغن)—get sandwiched in the middle before you finally state the action at the end.