I replaced the covers on my N80 and on reassembly my slider doesn't work i.e. doesn't answer calls or give the option to lock. The buttons work fine though. Anyone know what i've done?!

I had exactly the same problem when I replaced mine. I used a cheap, non-genuine part which looked exactly the same but the phone wouldn't recognise the slider closing. Put the old cover back on and all was well. Bought a genuine Nokia replacement cover and all works fine. The moral of the story escapes me but probably something to do with money.

If you're using a genuine part already then ignore the above. The buttons work okay as they rely on a completely different contact to the slider.

i can't open the top part any tips

Do you mean the front fascia? If so, you need a Torx screwdriver to take the two screws out then using something with a flat blade (preferably made of plastic) gently lever the fascia forward using the slider groove as leverage. Do each side at the top first then do one side lower down. The other, final, side shoudl then come off easily.

when you stay forward is it to the top of the phone or lift it up from the plastic?

There are any full disassembly instructions online that I've been able to find but if you do a search there are a few sites with pictures of how to remove the front of the phone. A picture being worth a thousand words and all that.

Try this one http://www.formymobile.co.uk/n80lcd.htm

i followed the instruction
i tried to lever it, however, it won't come loose, how much pressure is needed i don't want to break it

Well... I suppose the answer to this is...

... enough to do the job but not enough to break it.

If you're not happy about it probably best to take it to a Nokia Service Centre, will be worth it in the long run, as I know from bitterly expensive experience :embarasse

You have to leaver at two points on both sides. Towards the top and also towards the rest of the phone as close as possible. Once you have removed both clips on both sides, you need to slide the facia down to remove it from the phone,