how can we use wifi card in back track 4 pre final

Wireless support on Linux is iffy at best; and while it's one of the weakest spots supported on Linux, but that doesn't mean that it's impossible to get working either. You may need to just use a different wireless card, install ndiswrapper, or maybe BackTrack just doesn't work with that particular card, as sometimes is the case. For example, aircrack-ng has specific cards it works with (ones that can go into Promiscuous Mode, like some of the ones with the Atheros 512 or 512A chipset on them); so while I'm not giving you a step-by-step guide on how to fix this, I do believe I am at least pointing you in the right direction.

Use the tools available to you. Use dmesg, cat the /var/log files, try using a different live CD with Linux on it to see if wireless works with it instead. Does it? if it does, then maybe it's just BackTrack, or then again, maybe it's not. I think you need to dig a lot deeper into this.

One of my matras is "Follow the OSI Model". Way too many times people are troubleshooting Layer 4 (Transport) when Layer 1 (Physical) isn't working. If the wireless card isn't plugged in, then troubleshooting TCP/IP is worthless. Ya know what I mean? Be methodical. Be systematic. No, this isn't very fun, but if you don't follow the OSI Model, you are merely guessing and shooting hit and miss vs. taking it step-by-step.

Please post the output of

$ dmesg

if the device isn't detected (and assuming you correctly identified your adapter) you might want to download the drivers from the realtek site and compile them from source.

Please excuse me if you've already done this, but if not then it may be confusing matters.

BackTrack by default doesn't start any networking to prevent you announcing yourself on a network if you didn't want to.

It starts via this in the terminal:

/etc/init.d/networking start