Learn Swedish: Tjuven har stulit pengarna! - The thief has stolen the money!

Because it follows the helper verb har (has), it needs to be in the supine form. Stjäla is a Group 4 strong verb. Strong verbs often change their root vowel. Here, the ä changes to u. Additionally, all Group 4 verbs end in -it in the supine form, giving us stulit.

Is pengarna a plural word? In English, 'money' is singular.

Yes! Unlike in English, the Swedish word for money (pengar) is almost always treated as a plural noun. To say 'the money', you add the definite plural ending -na to get pengarna.

Could I say Tjuven stal pengarna instead?

Yes, but it changes the meaning slightly. Stal is the simple past tense ('stole'), used for a finished action in the past. By using har stulit ('has stolen'), you emphasize that the action has a result or connection to the present moment, just like in English.

How do I know if a noun like tjuv takes -en or -et for 'the'?