Learn Hardcore Norwegian: Barna går oppover bakken, mens moren ser nedover veien. - The children walk up the hill, while the mother looks down the road.

mens means while (simultaneous actions) or sometimes whereas (contrast). Here it’s “while.”
In Norwegian, you normally put a comma before a dependent clause introduced by mens, so the comma is expected.
Word order in a clause after mens is that of a subordinate clause: Subject comes before the verb (S–V–…), hence moren ser, not ser moren.
You can also front the mens-clause: Mens moren ser nedover veien, går barna oppover bakken. Note that when the subordinate clause comes first, the following main clause keeps the verb in second position: går barna … (V2).

Why is it moren and not mora? Are both correct?

Both are correct in Bokmål; they reflect two gender systems:

en mor – moren (common gender; more formal/neutral in writing)
ei mor – mora (feminine; common in speech and informal writing)

Pick one style and be consistent within a text. The sentence chooses the common-gender route: moren = “the mother.”

Why is it barna and not barnene? What are the forms of barn?

barn is an irregular neuter noun. Its forms in standard Bokmål are:

Singular: et barn (a child), barnet (the child)
Plural: barn (children), barna (the children)

Note: barnene is not standard Bokmål for this noun; use barna.

Does går mean “go” or “walk”? How do Norwegians say “are walking”?

å gå primarily means to walk (on foot). Don’t use it like English “go” for all movement.
For general travel/movement, use dra or reise (e.g., “go to Oslo” = dra til Oslo).
Norwegian doesn’t need a special continuous form: Barna går can mean “The children walk” or “The children are walking,” depending on context.

What’s the difference between opp, oppe, and oppover (and their “down” counterparts)?

opp = movement up to a point/level: Vi går opp.
oppe = location/state “up”: Vi er oppe.
oppover = movement upwards along a path/gradient (ongoing direction): Vi går oppover bakken.

Similarly:

ned (down, to a lower point), nede (down, located), nedover (downwards along a path): Hun ser nedover veien.

Why oppover bakken instead of just opp bakken? Are both okay?

Both are possible but with a nuance:

opp bakken focuses on reaching/going up the hill (endpoint-oriented).
oppover bakken emphasizes movement upward along the slope (direction/path). In many contexts they’re interchangeable; oppover sounds a bit more descriptive of the path.

Is ser nedover veien the normal way to say “looks down the road”? Why not ser på?

å se can mean both “to see” and “to look,” depending on context.
Use se på when you’re looking at a specific object/target: Hun ser på huset (she looks at the house).
With directional adverbs like nedover/bortover/utover, you typically use plain se: Hun ser nedover veien (“She looks down the road”). Alternatives with slightly different feel: kikker (peeks/glances), titter (peeks), speider (scans).

Is nedover veien idiomatic if the road isn’t actually sloping? Would bortover be better?

nedover veien literally suggests a downward direction. Many speakers also use it metaphorically as “down along the road.”
If there’s no sense of “down,” the neutral “along” is often bortover: ser bortover veien. So:
Sloping/“downward” feel: nedover veien
Neutral “along” on flat ground: bortover veien

Why are bakken and veien in the definite form?

Norwegian often uses the definite form for specific, contextually known things—just like English does in “up the hill, down the road.” Here, the speaker has a particular hill and road in mind:

bakken = “the hill/slope”
veien = “the road” You could say oppover en bakke / nedover en vei (“up a hill” / “down a road”) if you mean nonspecific ones.

Can I swap the clause order without changing the meaning?

Yes:

Original: Barna går oppover bakken, mens moren ser nedover veien.
Swapped: Mens moren ser nedover veien, går barna oppover bakken. Meaning is the same. Remember V2 in the main clause after a fronted subordinate clause: går barna …, not barna går … in that position.

How would I negate the sentence correctly?

Main clause: place ikke after the finite verb: Barna går ikke oppover bakken …
Subordinate clause (after mens): place ikke before the verb: … mens moren ikke ser nedover veien. Full example: Barna går ikke oppover bakken, mens moren ikke ser nedover veien.

Could I use gate/gaten instead of vei/veien? What’s the difference?

vei/veien = road/way (general, can be urban or rural)
gate/gaten (gata) = street (typically inside a town/city)
You’ll also see veg (Nynorsk/variant) instead of vei in some regions. So use veien for a road; use gata/gaten if you specifically mean a city street.

Any quick pronunciation tips for tricky parts?

går: long å, like “aw” in “law” (but rounded); final r is tapped/trilled in many dialects.
barna: the cluster rn often becomes a retroflex sound in Eastern Norwegian; both syllables are clear: BAR-na.
bakken: double k = a longer/stronger k; stress on first syllable: BAK-ken.
veien: ei is a diphthong like English “eye”: VAI-en.
nedover: typically NEH-do-ver; the d is pronounced.

Can I omit the verb in the second clause, like in English-style ellipsis?

No. Norwegian normally requires a verb in each clause. So

Ungrammatical: Barna går oppover bakken, mens moren nedover veien.
Correct: Barna går oppover bakken, mens moren ser nedover veien.