I got MKV files and i wanna put it on my sansa e280 but i dont know how to get it to change to the right format.

I’m not familar with that format. Where does the video come from? That will give a clue as to how to convert it.

MKV are “Matroska” files.

I’ve never tried it, but a Google search “mkv to fuze mp4” led to this page.

Im not familiar with it either my friend just sent me the file thats why im asking.

I told you what they were.

If you want to convert them, try a converter.  Here is one.