Sofie liet haar kin zakken en ik zag haar zweten toen de keuken te warm werd.

Liet is the simple past of laten.

• laten = to let, to have, to make, to cause
• liet = let / had / made in the past

So Sofie liet haar kin zakken literally works like Sofie let/made her chin drop. In smoother English, that is usually Sofie lowered her chin or Sofie dropped her chin.

Dutch often uses laten + infinitive where English might simply use one verb.

This is a causative construction:

• liet = caused/let
• zakken = to sink, drop, lower
• haar kin = her chin

So the structure is:

subject + laten (past: liet) + object + infinitive

In this sentence:

• Sofie = subject
• liet = caused/let
• haar kin = object
• zakken = infinitive

Literally: Sofie caused her chin to lower.

This kind of structure is very common in Dutch:

• Ik liet het glas vallen. = I dropped the glass.
  (literally: I let the glass fall)

No, the two haar forms have different jobs.

1. haar kin
   Here haar is a possessive determiner: her chin

2. ik zag haar zweten
   Here haar is an object pronoun: I saw her sweat / sweating

So even though both words are spelled haar, they function differently:

• haar = her in her chin
• haar = her in I saw her

Dutch does this just like English, where her can also be either possessive or object.

After verbs of perception like zien (to see), Dutch normally uses:

zien + object + infinitive

So:

• ik zag haar zweten = I saw her sweating / sweat

The verb after zag stays in the infinitive:

• zweten = to sweat / sweating in this structure

You do not use te here.

Compare:

• Ik hoorde hem zingen. = I heard him sing/singing.
• Ik zag haar lachen. = I saw her laugh/laughing.

So zweten is correct because it is part of a perception-verb construction.

Grammatically, it is an infinitive.

Dutch does not use a separate -ing verb form the way English does in sentences like I saw her sweating. Instead, Dutch uses the infinitive:

• ik zag haar zweten
• literally: I saw her sweat
• natural English: I saw her sweating

So even though English often uses -ing, Dutch usually just uses the basic verb form after zien, horen, etc.

Because toen here introduces a subordinate clause.

In Dutch subordinate clauses, the finite verb usually goes to the end.

Main clause:

• De keuken werd te warm.

Subordinate clause:

• toen de keuken te warm werd

That is standard Dutch word order:

• toen = when
• de keuken = the kitchen
• te warm = too warm
• werd = became

So the verb moves to the end because of the clause introduced by toen.

Werd te warm means became too warm.
Was te warm means was too warm.

That is an important difference:

• werd focuses on a change
• was describes a state

So:

• toen de keuken te warm werd = when the kitchen became too warm
• toen de keuken te warm was = when the kitchen was too warm

In your sentence, werd suggests the kitchen changed from acceptable to excessively warm.

Because warm is being used predicatively, not directly before a noun.

Compare:

• de warme keuken = the warm kitchen
  Here the adjective comes before the noun, so it usually gets -e

• de keuken werd te warm = the kitchen became too warm
  Here the adjective comes after the verb and describes the subject, so it stays warm, without -e

Also, after te (too), adjectives normally stay in the basic form:

• te groot
• te duur
• te warm

So te warm is exactly right.

Here toen means when.

Dutch toen can sometimes relate to past time in a way that English might translate as then, but in this sentence it is clearly a subordinating conjunction meaning when:

• toen de keuken te warm werd = when the kitchen became too warm

So here it introduces the time at which the sweating happened.

Because in Dutch, with verbs like zien, the normal pattern is:

subject + finite verb + object + infinitive

So:

• ik = subject
• zag = finite verb
• haar = object
• zweten = infinitive

That gives: ik zag haar zweten

This is the standard order after a perception verb:

• Ik hoorde hem praten.
• Ik zag hen rennen.

Putting zweten before haar would not be normal Dutch.

Yes, and the meaning is similar, but the structure and feel are a bit different.

1. ik zag haar zweten
   = I saw her sweating
   This is a direct perception construction. It feels immediate and visual.

2. ik zag dat ze zweette
   = I saw that she was sweating
   This uses a full subordinate clause with dat. It can sound slightly more explicit or descriptive.

Both are possible, but ik zag haar zweten is very natural when you directly observe the action happening.

The sentence is mainly in the simple past:

• liet = let/made/lowered
• zag = saw
• werd = became

This tense is very common in written Dutch storytelling and narration. English often does the same:

• Sofie lowered her chin and I saw her sweating when the kitchen became too warm.

So the sentence tells a sequence of past events in a narrative style.

Yes, it is understandable and natural enough, though it is somewhat literary or descriptive in tone.

It means that Sofie lowered/dropped her chin. Depending on context, that might suggest:

• tiredness
• worry
• resignation
• physical discomfort

Dutch often uses laten + infinitive for physical movements like this:

• zijn hoofd laten zakken = to lower/drop one’s head
• haar schouders laten zakken = to let her shoulders slump

So the phrasing fits well in descriptive narrative Dutch.