Learn Spanish (Latin America): Tenemos que comprar ropa para el invierno.

In Spanish, you generally need to use the definite article (el, la, los, las) when talking about seasons. So, 'for winter' becomes para el invierno (literally 'for the winter').

Why does the sentence use tenemos que comprar instead of just tenemos comprar?

The verb tener means 'to have' as in possession. When you want to express that you 'have to' or 'must' do something, you use the special phrase tener que followed by the unconjugated action word. That is why we say tenemos que comprar ('we have to buy').

Why is 'clothes' translated as ropa instead of the plural ropas?

In Spanish, ropa is treated as a singular noun, very much like the word 'clothing' in English. Even if you are buying several different items of clothing, you still just use the singular word ropa.

Why do we use para for the word 'for' in this sentence?

Para is used here because it shows the purpose of the clothes. You are buying the clothes specifically to be used during the winter season.