Learn Hardcore Dutch: Tom had geen paraplu, maar hij leende er één van de buurman. - Tom had no umbrella, but he borrowed one from the neighbor.

In Dutch, geen negates indefinite nouns (like “a/an” or “some”), whereas niet negates verbs, adjectives, adverbs, or definite noun phrases. Since paraplu here is an indefinite noun (“an umbrella”), you use geen. You would never say “Tom had niet een paraplu” in standard Dutch for “Tom didn’t have an umbrella.”

Why is there no indefinite article een before paraplu?

When you negate a noun with geen, it replaces the indefinite article een. So geen paraplu literally means “no umbrella” or “not an umbrella.” In positive sentences you’d say “Tom had een paraplu,” but once you negate it, een drops out: “Tom had geen paraplu.”

What does leende er één mean, and why are those separate words?

Leende is “borrowed” (past tense of lenen). Er here is a pronoun that replaces “from it/from that source.” Eén is a numeral pronoun meaning “one” (i.e. “one umbrella”). So leende er één literally means “borrowed one of them/from that source.”
Breakdown:

leende = borrowed
er = “of it” or “from there” (referring back to the concept of an umbrella)
één = one (i.e. one umbrella)

Could you just say “Tom leende een paraplu van de buurman” instead of “leende er één van de buurman”?

Yes, you can say “Tom leende een paraplu van de buurman.” That’s perfectly correct. Using “leende er één” instead of “leende een paraplu” is just more concise if you’ve already mentioned paraplu. It avoids repetition by using er + één.

Why is it van de buurman and not aan de buurman?

In Dutch, when you borrow something from somebody, you use van (“from”). Aan means “to” or “at,” which you’d use when you give something to someone: “Ik geef de bal aan de jongen.” Borrowing is the reverse action, so you use van: “Ik leen de bal van de jongen.”

Why is maar (“but”) used here instead of dus (“so”)?

Maar introduces a contrast or unexpected turn: “Tom didn’t have an umbrella, but he borrowed one…” There’s a slight surprise or contrast in not having one yet managing to get one.
Dus would simply indicate cause-and-effect: “Tom didn’t have an umbrella, so he borrowed one…” That’s also possible: “Tom had geen paraplu, dus leende hij er één van de buurman.” The meaning changes subtly: maar emphasizes contrast, dus emphasizes logical consequence.

Why de buurman instead of zijn buurman (“his neighbor”)?

In Dutch you often use the definite article plus the noun when the possessor is obvious from context. Since we already know it’s Tom’s neighbor, de buurman suffices. Zijn buurman is grammatically fine but redundant here.

Why are both verbs in the past tense (had, leende), and not in the present?

The original sentence describes a past situation—Tom lacked an umbrella at that moment and then borrowed one. So you use past-tense forms:

had (past of hebben)
leende (past of lenen)
If you wanted to state it as a habit or general fact, you could switch to present: “Tom heeft geen paraplu, maar hij leent er één van de buurman.”