Learn Dutch: Deze steen weegt vijf karaat. - This stone weighs five carats.

In Dutch, units of measurement, weight, and time usually remain in the singular when preceded by an exact number. Just like you say drie kilo (three kilos) or twee meter (two meters), it is vijf karaat.

What are the past tense and past participle forms of the verb wegen?

Wegen (to weigh) is a strong verb. The simple past is woog (plural: wogen) and the past participle is gewogen.

This lesson focuses on dative complements with evaluative adjectives. How would I use that grammar to say "This stone is too heavy for me"?

You would say Deze steen is mij te zwaar. In that sentence, mij acts as the dative complement to the evaluative adjective phrase te zwaar (too heavy), indicating for whom the evaluation holds true.

What happens to the spelling of steen in the plural?

The plural is stenen. Notice that the double ee becomes a single e because it is now in an open syllable (ste-nen), but it keeps the same long vowel sound.