Hon hällde upp te ur termosen medan jag lade nötter i en liten skål.

Because hälla upp is the usual Swedish expression for pouring a drink into a cup, glass, or similar container.

• hälla = to pour
• hälla upp = to pour out / pour into a serving container or cup

So:

• Hon hällde te can sound incomplete or less natural in this context.
• Hon hällde upp te sounds like She poured some tea in the normal everyday sense.

Swedish often uses verb + particle combinations like this, and the particle can change the meaning or make it more idiomatic.

Here ur means out of.

• ur termosen = out of the thermos

This is used because tea is being poured from inside the thermos.

Compare:

• ur = out of, from inside
• från = from, away from, from a person/place/source more generally

So in this sentence, ur is the natural choice because the tea comes from inside the thermos.

Termosen is the definite form, meaning the thermos.

Swedish nouns often add an ending to show the:

• en termos = a thermos
• termosen = the thermos

So ur termosen means out of the thermos, not just out of a thermos.

Both lade and la can be used as the past tense of lägga (to lay / put).

• lade is often considered the more formal or standard full form
• la is very common in everyday speech and writing

So:

• jag lade nötter i en liten skål
• jag la nötter i en liten skål

Both are acceptable. A learner will often meet both.

Swedish often leaves out a word like some where English might use it.

So:

• lade nötter i en liten skål literally looks like put nuts in a small bowl
• but it naturally means put some nuts in a small bowl

This is very common in Swedish. Plural nouns without an article often have a general or indefinite meaning.

Because the nuts are being placed inside the bowl, not on it.

• i = in, inside
• på = on, onto

So:

• i en liten skål = in a small bowl
• på en liten skål would mean on top of a small bowl, which does not fit here

In Swedish, adjectives change form depending on the noun phrase.

Here, en liten skål is an indefinite singular common-gender noun phrase:

• en = a
• liten = small
• skål = bowl

So liten is the correct form here.

Compare:

• en liten skål = a small bowl
• den lilla skålen = the small bowl

When the noun is definite, the adjective usually changes, so then you get lilla.

Because medan means while, and it introduces a subordinate clause. In this sentence, the subordinate clause begins with medan, followed directly by the subject jag:

• medan jag lade nötter i en liten skål

That is normal subordinate clause word order in Swedish.

A useful thing to notice is that the finite verb comes after the subject here:

• medan jag lade...

If there were a sentence adverb like inte, it would usually come before the verb in the subordinate clause:

• medan jag inte lade...

This is a common feature of Swedish subordinate clauses.

Usually, yes. Medan commonly means while and shows that two actions were going on at the same time.

Here:

• Hon hällde upp te
• medan jag lade nötter i en liten skål

This gives the sense that these actions happened simultaneously.

In some contexts, medan can also express contrast, more like whereas, but here it clearly means while.

Just like with nötter, Swedish often does not use a separate word where English might say some.

• hällde upp te = poured tea / poured some tea

This is perfectly natural in Swedish. Mass nouns like te often appear without an article.

Compare:

• dricka te = drink tea
• köpa kaffe = buy coffee
• hälla upp te = pour tea

Yes. Skål is the normal word for bowl.

So:

• en skål = a bowl
• en liten skål = a small bowl

Be aware that skål! can also mean cheers! when making a toast, but in this sentence it clearly means the container.

A very natural spoken version might be:

• Hon hällde upp te ur termosen medan jag la nötter i en liten skål.

The main difference is la instead of lade, since la is very common in speech.

You might also hear the sentence broken up with natural pauses in speech, but the original version is already completely correct and natural.