Learn Finnish: Todistaja piti laukusta kiinni kynsin hampain. - The witness held on to the bag tooth and nail.

The instructive case is formed by adding the suffix -in to the plural stem of a noun. It expresses doing something "by means of" or "using" an instrument or body part. Here, kynsi (nail) becomes kynsin and hammas (tooth) becomes hampain. In modern Finnish, this case is mostly found in fixed idioms like this one.

Why is the bag in the elative case (laukusta)?

The elative case (-sta/-stä) is required by the phrasal verb pitää kiinni (to hold on to). Whenever you hold on to something in Finnish using this verb, the object you are holding must take the elative ending.

Does the Finnish idiom literally translate to "tooth and nail"?

Almost exactly! It translates literally to "with nails, with teeth." Finnish just reverses the order of the words compared to the English idiom, but the core meaning of holding on or fighting fiercely is exactly the same.

How does the word hammas change its stem to become hampain?

Words ending in -as like hammas (tooth) change their -s to -aa- in the stem, giving hampaa-. For the plural instructive, we add the plural -i- and the instructive -n. The double vowel -aa- drops one -a- before the plural -i-, resulting in hampain.