Learn Hardcore Dutch: Wanneer de zonnestralen door het raam vallen, lijkt de nieuwe trui nog warmer. - When the sunbeams fall through the window, the new sweater seems even warmer.

wanneer translates as “when” in a temporal sense, pointing to a specific moment or period. You use wanneer for single events, questions (e.g. Wanneer kom je?), and in formal contexts.
als can also mean “when,” but it’s typically used for repeated or habitual situations (e.g. Als ik thuis kom, zet ik de tv aan) or as “if” in conditional clauses. In our sentence, wanneer highlights the particular moment the sun’s rays fall through the window.

Why is the verb vallen at the very end of the clause wanneer de zonnestralen door het raam vallen?

Because this is a subordinate clause introduced by wanneer. In Dutch subordinate clauses, the finite verb goes to the end (SOV word order). The breakdown is:

conjunction: wanneer
subject: de zonnestralen
rest of the clause: door het raam
finite verb: vallen

Do I need that comma after the subordinate clause?
wanneer de zonnestralen door het raam vallen, …?

Yes. When a subordinate clause (bijzin) precedes the main clause (hoofdzin), you separate them with a comma. If you start with the main clause, the comma is optional or often left out:

Main first: De nieuwe trui lijkt nog warmer wanneer de zonnestralen door het raam vallen.
Subordinate first: Wanneer de zonnestralen door het raam vallen, lijkt de nieuwe trui nog warmer.

Why does the main clause start with lijkt instead of de nieuwe trui? Why is the verb before the subject here?

After a fronted subordinate clause, Dutch requires inversion in the main clause: the finite verb comes immediately after the comma, before the subject. So the order becomes:

subordinate clause + comma
finite verb (lijkt)
subject (de nieuwe trui)
remainder (nog warmer)

What exactly does lijkt mean in this context, and why not just use is?

lijkt means “seems” or “appears,” expressing a subjective impression. Using is would assert an objective fact (“the sweater is warmer”), whereas lijkt leaves room that it only feels or looks warmer when the sun shines on it.

What role does nog play in nog warmer?

Here nog functions as “even,” intensifying the comparative. So nog warmer means “even warmer.”

How do you form the comparative warmer from warm?

Most Dutch adjectives form the comparative by adding -er to the base adjective:
• warm → warm + -er = warmer
Note: adjectives ending in -r often double the -r (e.g. dik → dikker).

Why is the adjective nieuwe written with an -e before trui?

Because nieuwe is preceded by the definite article de. Adjectives take a weak inflection (add -e) when they come before a noun with de, het, a demonstrative, or a possessive. For example:
• de trui → de nieuwe trui
By contrast, an indefinite neuter noun would use the uninflected form: een nieuw huis.

Why do we say de zonnestralen and de nieuwe trui, but het raam?

Dutch has two main noun genders marked by articles:
• de for common gender (masculine/feminine) and all plurals
• het for neuter singular
In this sentence:

de zonnestralen (plural → always de)
de nieuwe trui (common-gender singular → de)
het raam (neuter singular → het)
You often have to memorize which singular nouns are neuter and which are common.