Learn Hardcore Dutch: De sneeuwpop smolt langzaam toen de zon sterker werd. - The snowman melted slowly when the sun became stronger.

Dutch has two definite articles: de (common gender) and het (neuter). sneeuwpop (snowman) is treated as a common-gender noun, so it takes de. Unfortunately, gender assignment is largely arbitrary in Dutch, so you often have to learn it with each new noun or check a dictionary.

How is the past tense smolt formed from smelten? Are there irregularities?

smelten is a strong (irregular) verb. In the past singular third-person, the vowel changes and you get smolt. In the past plural it’s smolten. The past participle is gesmolten, used with hebben or zijn.

Why is the adverb langzaam placed after the verb smolt? Could we move it?

In a main clause, Dutch typically follows V2 word order: the finite verb in second position, then modifiers like adverbs. So you get De sneeuwpop smolt langzaam…. You can also front the adverb for emphasis:
Langzaam smolt de sneeuwpop…
but you generally wouldn’t stick it in the middle of the verb phrase (e.g. sloft langzaam is ungrammatical here).

When do we use toen vs wanneer in Dutch?

toen is used for single events in the past (one-off situations).
wanneer is used for repeated events, questions or to talk about present/future time:
– Toen ik jong was, speelde ik veel. (one past event)
– Wanneer ga je morgen naar school? (question/present-future)

Why is werd at the end of the clause toen de zon sterker werd?

Any subordinate clause introduced by a conjunction like toen moves the finite verb to the end. So you get subject (de zon), object/complement (sterker), then finite verb (werd).

How do you form the comparative adjective sterker?

For most monosyllabic adjectives, add -er: sterk → sterker.
If an adjective ends in a vowel + single consonant (e.g. gat → gatter doesn’t work!), you usually double the consonant first or adjust spelling. Longer adjectives often use meer: meer interessant.

Could we express this sentence in the present perfect instead of the simple past?

Yes. You’d use the auxiliary zijn or hebben + past participle:
De sneeuwpop is langzaam gesmolten toen de zon sterker werd.
Dutch often uses simple past for narration, but present perfect is fine if you want to emphasize the result.

What about using nadat instead of toen?

You can, but time sequencing changes: nadat requires the earlier action to be in the past perfect:
De sneeuwpop smolt langzaam nadat de zon sterker was geworden.
Here, sterker was geworden is the past perfect of worden (“to become”), because nadat establishes a clear before-after.