For newby, i would recommend to put all the output into your next post for maemo users to take a look at it. Lots of knowledge here.

Explanationof the commands:
'mount' gives the information about the partitions mounted to your system,
'df' shows the free space of the mounted partitions,
'du -kx / | sort -n | tail -n 20' sorts the 20 largest folders of the device to look for strange sizes,
'sudo gainroot' gives the right of 'root' user,
'sfdisk -l' prints out the partion table(s) of the device(s) of the N900
You are looking for references to /dev/mmcblk0p2, for /home and for /opt, aka. /home/opt.
None of the commands do any harm, they help to fetch some information about the device.

There might be more information necessary in the process to debug the issue...

Btw. If you 'formatted' your N900, what exactly did you reference to?