Farbrorn hällde upp te ur termosen medan barnen åt kex.

Both are in the simple past.

• hällde = past of hälla = to pour
• åt = past of äta = to eat

So the sentence describes something that happened in the past.

Because hälla upp is a very common Swedish verb phrase meaning to pour out or to serve by pouring, especially into cups or glasses.

• hälla = to pour
• hälla upp = to pour out / pour some for serving

So hällde upp te sounds natural for poured tea in the sense of serving it.

Here ur means out of or from inside.

So:

• ur termosen = out of the thermos

Swedish often uses ur when something comes from inside a container.

Compare:

• ur flaskan = out of the bottle
• ur väskan = out of the bag

från is a more general from, but ur is more precise here because the tea is being poured from inside the thermos.

Those are definite forms in Swedish. Instead of usually using a separate word like the, Swedish often puts definiteness at the end of the noun.

Examples in this sentence:

• farbrorn = the uncle / the old man
• termosen = the thermos
• barnen = the children

So the endings are doing the job that the does in English.

Because Swedish often uses no article with:

1. mass nouns like te
2. indefinite plurals like kex

So:

• te = tea
• kex = biscuits/crackers, in an indefinite plural sense

This is similar to English, where you can say:

• drink tea
• eat biscuits

without needing a or the.

Literally, farbror traditionally means father’s brother, so uncle.

But in real Swedish, farbror can also be used more loosely for an older man or the old gentleman, especially in stories or descriptive narration.

So depending on context, farbrorn may mean:

• the uncle
• the older man
• the gentleman

If the meaning has already been given to you, that tells you which one is intended here.

Because barnen is the definite plural form: the children.

Here is the pattern:

• ett barn = a child
• barnet = the child
• barn = children
• barnen = the children

So barnen shows that we are talking about specific children, not children in general.

Yes. åt is the past tense of äta.

This is an irregular verb, so the form changes a lot:

• att äta = to eat
• äter = eat / eats
• åt = ate
• ätit = eaten

English has the same kind of thing with irregular verbs:

• eat
• ate
• eaten

So åt may look surprising, but it is a normal irregular past form.

medan means while.

It introduces a subordinate clause:

• medan barnen åt kex = while the children ate biscuits/crackers

This tells you that the two actions happened at the same time:

• the man/uncle was pouring tea
• the children were eating kex

So medan links simultaneous actions.

Yes. That is a very important Swedish word-order point.

In the original sentence:

• Farbrorn hällde upp te ur termosen medan barnen åt kex.

If you move the medan clause to the front, the main clause changes word order:

• Medan barnen åt kex hällde farbrorn upp te ur termosen.

Notice:

• not farbrorn hällde
• but hällde farbrorn

This happens because Swedish main clauses follow the verb-second rule.

So when something else comes first, the finite verb still has to come in second position.

Because kex is one of those Swedish nouns whose plural often looks the same as the singular.

Typical forms:

• ett kex = a biscuit/cracker
• kex = biscuits/crackers
• kexet = the biscuit/cracker
• kexen = the biscuits/crackers

So in åt kex, the form kex is plural or general/indefinite from context, even though it looks the same as the singular base form.