Learn Farsi: ما هر جمعه خرید می‌کنیم.

This is a classic Farsi compound verb! Instead of having a single unique verb for "to shop", Farsi pairs the noun خرید (shopping) with the helper verb کردن (to do). When conjugated for "we" in the present tense, کردن becomes می‌کنیم.

Since it's a regular habit, why doesn't the noun جمعه (Friday) have a plural ending?

Just like in English, the word هر (every or each) must always be followed by a singular noun. So you say هر جمعه (every Friday), never "every Fridays".

How does the verb می‌کنیم show that this is a repeated habit?

The prefix می‌ attached to the verb shows that an action is ongoing or habitual. While this tense can also mean "we are doing" right now, the time phrase هر جمعه (every Friday) makes it clear that this is a regular habit.

The English translation says "we do the shopping." Where is the word for "the" in the Farsi sentence?

Farsi doesn't have a direct equivalent for the definite article "the". Nouns are usually assumed to be definite based on the context unless you add an indefinite marker, like the word یک (a/one). Here, خرید naturally translates to "the shopping" in English.