Questions & Answers about Ein Monat hat vier Wochen.
Why is it "Ein Monat" and not "Eine Monat"?
Monat is a masculine noun (der Monat). Since it is the subject of the sentence (in the nominative case), the word for "a" or "one" is ein. You would only use eine for feminine nouns, like eine Woche.
How does the plural work for "Wochen"?
Is "vier Wochen" in the accusative case because of the verb "haben"?
Yes, exactly! The verb haben (to have) always requires its direct object to be in the accusative case. While the number vier does not change its ending to show the case, the phrase vier Wochen is grammatically accusative here.