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In modern Finnish, many of its instrumental uses are being superseded by the adessive case, as in "minä matkustin junalla" -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."
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 case. There is no singular version of this case
|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|