Learn Finnish: Luottavaisin mielin lapsi hyppäsi isän syliin. - Confidently, the child jumped into father's arms.

This phrase uses the instructive case, indicated by the -in ending. The instructive describes how or by what means an action is done. A quirky feature of this case is that it is almost exclusively used in the plural form, even when referring to a single person's action. Here, it translates literally to "with confident minds."

Could I just use the regular adverb luottavaisesti instead of luottavaisin mielin?

Yes, lapsi hyppäsi luottavaisesti is grammatically correct and means the same thing. However, Finnish heavily relies on fixed mielin (with minds or moods) phrases for literary or expressive descriptions of emotions. Using the instructive case here sounds highly natural and idiomatic in storytelling.

How do you form the instructive adjective luottavaisin from luottavainen?

Adjectives must agree with the noun they modify, so luottavainen needs to be in the instructive plural to match mielin. First, find the plural stem by changing the -nen ending to -si-, giving you luottavaisi-. Then, simply add the instructive suffix -n to get luottavaisin.

Why is "into father's arms" translated as isän syliin? Doesn't syli mean "lap"?