Learn Swedish: Vi köper ofta frukt på marknaden på fredagar. - We often buy fruit at the market on Fridays.

Combining på with the plural form of a weekday, like på fredagar, is how you express a recurring habit in Swedish ("on Fridays" or "every Friday"). If you used the singular form på fredag, it would mean just "this coming Friday".

Why is the word på used twice in this sentence?

It is just a coincidence that Swedish uses the same preposition for both the location and the time here! The first på means "at" (på marknaden = at the market). The second på is the time preposition used with days of the week (på fredagar = on Fridays).

Can I say Vi ofta köper to match the English word order "We often buy"?

No, because Swedish strictly follows the "V2 rule" in main sentences, meaning the verb must be the second piece of information. Since Vi (We) is the first piece, the verb köper (buy) has to come immediately next. Adverbs like ofta (often) are usually placed right after the verb.

What is the base word for marknaden?

The base word is en marknad (a market). Because it is an en-word, we add the suffix -en to the end to make it "the market": marknaden.