Learn Italian: Potresti comprare il filo di cotone che è nel negozio? - Could you buy the cotton thread that is in the shop?

Potresti is the present conditional form of potere (can/to be able to), meaning "could you". While you could use the regular present tense puoi ("can you"), using the conditional potresti makes the request much softer and more polite, just like in English.

Why isn't comprare conjugated to match "you"?

Because it immediately follows the modal verb potresti. In Italian, when you have a modal verb expressing ability, desire, or obligation (like potere, volere, or dovere), the main action verb that comes after it always stays in its infinitive (dictionary) form.

Why is it filo di cotone and not just "cotone filo"?

In English, you can use a noun like "cotton" as an adjective right before another noun to make "cotton thread". In Italian, you can't stack nouns like this. Instead, you describe what something is made of by using the noun + di (of) + the material.

Why does the sentence use nel negozio rather than just in il negozio?