I issue the command df and recive the folowing output:

root@juanlinux root]# df
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/hda2 18937836 3541476 14434352 20% /
/dev/hda1 51206 8966 39596 19% /boot
none 127384 0 127384 0% /dev/shm
I have only one HD on the system so way hda1 and hda2
and how I know what is the total space free on the hole HD?
thanks

you should try
#df -h
df tells you these things
Filesystem = Partition hda1 is first parition hda2 second primary partition.

rest is self explanatory
size of the partition
used part
Free Available
Used Percentage
Mount points

And one hard drive is correct. The numbers represent partitions on the single hard drive (hda).

If you had multiple hard drives, you'd see things like hdb and hdc etc.

Some distros will create several partitions during the install if you run a wizard or something.

The -h option from manpreetnehra means "human readable". Most of us normal people (all super geeks excluded ) would like to read things in a comprehensive form. So instead 1024 bytes, we like to see 1 Kilobyte. And so on, the -h option does this for you. If you have 1024000 then it will display this as 1MB (might be wrong on that, but I hope you see what I mean).

Cool