The instructive

1. Use

The instructive has the basic meaning of "by means of". The case is mostly used in the plural. It is a comparatively rarely used case, though it is found in some commonly used expressions, such as "omin silmin" (= with one's own eyes) and "omin käsin" (= with one’s own hands).

In modern Finnish, many of its instrumental uses are being superseded by the adessive case, as in "minä matkustin junalla" -> "I traveled by train."

2. Formation

The characteristic ending of the instructive is -in. The ending gets added to the weak plural stem of the word, which you find by putting the word in the plural inessive case. There is no singular version of this case

Finnish English
Menin sinne junin ja autoin. I went there by train and car.
Jatkan uusin voimin. I continue with new strength.
Tulitko tyhjin käsin? Did you come empty-handed?
molemmin puolin on both sides
aamuin, illoin, öin in the morning, evening, night
kaikin puolin in all respects
monin tavoin in many ways
kaikin keinoin in every possible way