Når solen går ned, hælder vi te fra en termokande og ser på søen.

Yes. Når means when here.

A very common difference for English speakers is:

• når = when in a general, repeated, or future-type sense
• da = when about one specific event in the past

So in this sentence, Når solen går ned suggests something like when the sun goes down in general, or whenever the sun goes down, not one single past occasion.

Examples:

• Når det regner, bliver vi inde. = When it rains, we stay inside.
• Da solen gik ned, blev det koldt. = When the sun went down, it got cold.
  This refers to one specific past moment.

Because solen means the sun.

In Danish, the definite article is often added to the end of the noun:

• en sol = a sun
• solen = the sun

So:

• sol = sun
• solen = the sun

Since English also says the sun, this matches nicely here.

Går ned literally means goes down, but in this context it is the normal way to say sets about the sun.

So:

• solen går ned = the sun goes down / the sun sets

This is a very natural Danish expression.

Related expressions:

• Solen står op. = The sun rises.
• Solen er gået ned. = The sun has set.

Because Danish uses verb-second word order in main clauses.

The sentence begins with the subordinate clause:

• Når solen går ned

After that, the main clause starts, and in a Danish main clause the finite verb normally comes in the second position. That is why you get:

• Når solen går ned, hælder vi te ...

not

• Når solen går ned, vi hælder te ...

This is one of the most important word-order patterns in Danish.

Compare:

• Vi hælder te fra en termokande.
• Om aftenen hælder vi te fra en termokande.
• Når solen går ned, hælder vi te fra en termokande.

In all of these, the finite verb takes the second slot in the main clause.

Hælder is the present tense of at hælde, which means to pour.

So:

• at hælde = to pour
• hælder = pour / pours / are pouring, depending on context

In Danish, the present tense is often used where English might use either simple present or present continuous.

Examples:

• Jeg hælder te op. = I pour tea / I am pouring tea.
• Vi hælder te fra en termokande. = We pour tea from a thermos flask.

Fra means from, and en termokande means a thermos flask / a vacuum flask.

So:

• fra = from
• en termokande = a thermos flask

Danish normally uses fra with pour from:

• hælde te fra en termokande = pour tea from a thermos flask

About termokande:

• termo- relates to heat
• kande means jug / pot

So the whole word literally suggests a container that keeps things warm.

Because en termokande means a thermos flask, while termokanden means the thermos flask.

The sentence uses the indefinite form, so it refers to a thermos flask without specifically identifying it.

• en termokande = a thermos flask
• termokanden = the thermos flask

If the sentence had fra termokanden, it would mean from the thermos flask.

Because both verbs share the same subject: vi.

The structure is:

• hælder vi te fra en termokande
• og ser på søen

The second verb ser is linked by og and keeps the same subject, so Danish does not need to repeat vi.

In English, this works the same way:

• we pour tea ... and look at the lake

You could repeat the subject for emphasis in some contexts, but normally you would not.

Because at se på means to look at, while at se by itself often means to see.

This is an important distinction:

• at se = to see
• at se på = to look at / watch

So:

• Vi ser søen. = We see the lake.
• Vi ser på søen. = We look at the lake.

In your sentence, the idea is not just that the lake is visible, but that they are actively looking at it.

Søen means the lake.

Like solen, it uses the Danish definite ending:

• en sø = a lake
• søen = the lake

So:

• ser på søen = look at the lake

These letters are very common in Danish and often feel unfamiliar at first.

A rough guide:

• å in når and går sounds somewhat like the vowel in English more or or, but the exact Danish sound is different.
• ø in søen has no exact English equivalent. It is a front rounded vowel. A rough approximation is something between the vowel in French deux or German schön, if you know those.

A few notes:

• når and går rhyme in Danish
• sø has a different vowel from both sol and ser

It is best to learn these by listening, because English does not have perfect matches for Danish ø and å.

It most naturally sounds habitual or scene-setting: something like when the sun goes down, we pour tea from a thermos flask and look at the lake.

Because of når, the sentence can suggest:

• something that usually happens
• a repeated routine
• a descriptive scene

If you wanted one specific past event, Danish would more naturally use forms like:

• Da solen gik ned, hældte vi te fra en termokande og så på søen.

That would mean:

• When the sun went down, we poured tea from a thermos flask and looked at the lake.

Sometimes yes, but look at is safer here.

• se på can mean look at
• in some contexts it can also mean watch, especially with TV, a film, or people doing something

For a lake, look at the lake sounds most natural in English.

So:

• ser på søen = look at the lake

not usually:

• watch the lake, unless you specifically mean observing it closely for some reason.