Learn Spanish (Spain): Vuestro abuelo tiene un coche nuevo. - Your plural grandfather has a new car.

In Spanish, possessive words like vuestro must match the thing being owned, not the owners. Since there is only one grandfather (abuelo), we use the singular vuestro. If you were talking about multiple grandfathers, it would be vuestros abuelos.

I thought "your" for multiple people was su. Why use vuestro here?

In Spain, vuestro is used for the plural informal "your" (like saying "you guys' grandfather"). You would only use su for the plural "your" if you were speaking formally to a group of people using ustedes.

If vuestro refers to multiple owners, why is the verb tiene instead of tenéis?

The verb always matches the person or thing doing the action (the subject). Here, the grandfather (abuelo) is the one who has the car. Since the grandfather is a single person (he), we use tiene. You would use tenéis if the sentence was "You all have a car" (Vosotros tenéis un coche).

Why does nuevo come after coche?

In English we put descriptive words (adjectives) before the noun, like "new car". In Spanish, adjectives usually come right after the noun they describe, which is why it is coche nuevo (literally "car new").