Yes, it's instructive case (
viisiütlev kääne). It looks like genitive, but answers to the question
how?.
And most of the time it uses plural form: "paljajalu ~ palja jalu" ~
barefootedly It's pretty much extinct in Estonian, except in fixed phrases:
"ummisjalu" - I don't think I'm able to translate that
"paljapäi ~ palja päi" ~
bareheadedly "ammuli sui" ~
fully-opened-mouthedly "kinnisilmi ~ kinni silmi" ~
closed-eyedly etc...
Although you can also form them yourself. But the plural form doesn't use the regular "-de/te" ending like genitive plural, but vowel plural, and in the same grade as genitive.
For example, if you have partitive "jalga" (strong grade) and you can form vowel plural partitive "jalgu", then from genitive "jala" (weak grade) you can form "jalu":
sõrme ->
sõrmi (NB: weak grade, not like partitive)
kõrva ->
kõrvu (NB: weak grade, not like partitive)
But I don't think this works with words whose vowel plural is marked with multiple vowels like "
ai",
ei and such. Although, I have seen present participle (-v) instructive form "-vi", even though its partitive plural is "-vaid":
"värisevi sõrmi" (~ "värisevate sõrmedega, sõrmed värisemas") ~
with shaking fingers"lipendavi kõrvu" (~ "lipendavate kõrvadega, kõrvad lipendamas") ~
with flapping earsBut it seems that in some dialects the grade differs from genitive, and is usually strong. It seems that even "jalgsi" is instructive. And then there are also parallel forms like "piki ~ pikki" (
~along), "silmi
(weak) ~ silmi
(strong)", etc...
Although, instructive case is pretty much dead, you can find it everywhere if you know how to look for it and compare words with Finnish. For example "nii" (
so, like this, this way; Finnish
niin) and "kaua" (
for a long time; Finnish
kauan) are instructive forms.
Some examples of instructive case in different dialects and languages.