Learn Spanish (Latin America): Ella compra un kilo de papas en el supermercado. - She buys a kilo of potatoes at the supermarket.

You are currently learning to use a (to) with the verb ir (to go) for destinations. However, because she is not going to the supermarket but is already there doing an action, we use en (in or at). Use a for where you are traveling to, and en for where you currently are.

I thought the Spanish word for potato was patata. Why does this sentence use papas?

In Latin American Spanish, the word for potato is papa (plural: papas). The word patata is primarily used in Spain.

Why is there a de in un kilo de papas?

In Spanish, when you talk about a quantity or measurement of something, you connect the amount (un kilo) and the item (papas) with the preposition de (of). It literally translates to 'a kilo of potatoes'.

Could we just say Compra un kilo... and leave out Ella?

Yes! In Spanish, you can often drop the subject pronoun because the verb ending tells you who is doing the action. However, because compra can mean 'he buys', 'she buys', or 'you buy' (formal), including Ella makes it perfectly clear that 'she' is the one buying.