Learn Finnish: ”Viisikymmentä tuhatta!” huudahti keräilijä ja nyökkäsi epäröimättä. - "Fifty thousand!" the collector exclaimed and nodded without hesitation.

This is the inversion used in reported speech. When the reporting clause (the part that tells us who is speaking) comes directly after a quotation, Finnish flips the usual Subject-Verb word order to Verb-Subject. If the quote came after the reporting clause, it would use standard word order: Keräilijä huudahti: 'Viisikymmentä tuhatta!'

Why is the word for thousand, tuhatta, in the partitive case?

In Finnish, any number greater than one in the nominative case requires the word following it to be in the partitive singular. Here, the number viisikymmentä (fifty) forces the next number word, tuhat (thousand), into its partitive singular form, tuhatta.

How is the verb huudahtaa different from the basic verb huutaa (to shout)?

Huudahtaa is a momentane verb derived from huutaa. The -ahtaa / -ähtää ending indicates a sudden, quick, or one-time action. While huutaa generally means 'to shout' or 'to yell,' huudahtaa specifically means 'to exclaim' or 'to let out a short shout.'

What grammatical form is epäröimättä (without hesitation)?

This is the abessive form of the third infinitive, often called the -mättä / -matta form. It is formed from the verb epäröidä (to hesitate) and translates to 'without doing' the action. Here, it simply means 'without hesitating.'