Learn Hardcore Norwegian: Jeg så en gråtende baby på bussen, og moren byttet bleie i et stille hjørne. - I saw a crying baby on the bus, and the mother changed a diaper in a quiet corner.

Jeg is the subject form (I) and is used because it’s the person doing the seeing: Jeg så ... = I saw ....
Meg is the object form (me) and would be used after a verb or preposition when you are the one being acted on, e.g. Hun så meg (She saw me) or for meg (for me).

Why is the past tense of å se written så? Is that the same as så meaning so/then?

Here så is the past tense of å se (to see):

infinitive: å se
past: så
past participle: sett

It’s a different word from så meaning so/then, even though it’s spelled the same. Context tells you which one it is.

What does en gråtende baby mean grammatically—why gråtende and not gråter?

gråtende is the present participle (roughly crying / weeping used like an adjective). It describes the noun baby.
So: en gråtende baby = a crying baby.

gråter is the present tense verb form (cries/is crying) and normally needs a subject: Babyen gråter = The baby is crying. You don’t use gråter directly as an adjective before a noun.

Why is it en gråtende baby and not et?

Because baby is a common-gender noun in Norwegian (in Bokmål), so its indefinite article is en: en baby.
Neuter nouns take et (e.g. et barn).

Why is it på bussen and not i bussen?

Norwegian commonly uses på with public transport vehicles when you mean on (board) the bus/train/plane:

på bussen, på toget, på flyet

i bussen can occur, but it often emphasizes being physically inside the bus (or contrasts with being outside). In everyday “I was riding the bus,” på bussen is the normal choice.

What is the difference between og and også, and does the comma matter here?

og means and and links two clauses: ..., og moren ...
også means also/too and would change the meaning: ..., og moren også ... = ..., and the mother also ...

The comma is standard before og when it connects two full clauses (each with its own subject and verb):

Clause 1: Jeg så ...
Clause 2: moren byttet ...

Why does it say moren (definite) and not en mor (indefinite)?

Because the mother is understood as the baby’s specific mother, so Norwegian naturally uses the definite form: moren = the mother (i.e., the baby’s mother). This is a common pattern when someone is uniquely identifiable from context (family relations, body parts, etc.).

How is the definite form moren built?

Norwegian often attaches the definite article as a suffix. For mor (mother):

indefinite: en mor
definite: mora or moren

Both are common in Bokmål; mora is often more colloquial, moren can feel a bit more formal/neutral.

Why is the verb byttet and not bytte?

byttet is past tense (preterite) of å bytte (to change): byttet = changed.
bytte is the infinitive (to change) and would need an auxiliary or a different structure, e.g. skal bytte (will change) or å bytte (to change).

Is bleie masculine or feminine or neuter—and why no article in byttet bleie?

bleie is usually common gender in Bokmål: en bleie. (Some speakers may use feminine ei bleie as well.)

In byttet bleie, Norwegian often omits the article in certain fixed, everyday expressions, especially for actions involving clothing/body-care items (similar to English change diapers / change clothes). You could also say byttet en bleie if you want to emphasize a (single) diaper, but the article-less version sounds very natural.

Why is it i et stille hjørne—why both et and stille?

hjørne is neuter, so it takes et: et hjørne = a corner.
stille is an adjective meaning quiet and must agree in form with a neuter noun in the indefinite singular, which usually means adding -t:

common gender: en stille plass
neuter: et stille hjørne

Is the word order in the second part fixed? Why is it og moren byttet ... and not og byttet moren ...?

In a normal main clause, Norwegian uses Subject–Verb order: moren byttet ...

byttet moren ... would be possible only in special cases, mainly after something is moved to the front of the clause (triggering inversion), e.g.

Der byttet moren bleie. (There the mother changed a diaper.)
Here Der is first, so the verb comes second, and the subject comes after the verb. But after og with a regular clause, moren byttet ... is the default.

Could this sentence be interpreted as two separate actions happening at different times? Does og imply sequence?

og mainly means the actions are both true; it doesn’t strictly say which happened first. In real-life interpretation, many people will infer a sequence (you noticed the baby, and meanwhile/then the mother changed the diaper), but the grammar itself is neutral.

If you want clearer sequencing, you might use:

så (then): ..., og så byttet moren ...
deretter (afterwards): ..., og deretter byttet moren ...