I trim the nails when I can get them to hold still. Mama cat religiously uses her scratchy box.

A cat's nail is sharp and pointy. When you trim the pointy part off of he end, they can scratch but it doesn't do as much damage.

You can train them to not scratch furniture and carpets. My brother has successfully done this with his cat Max. When Max was a young cat my brother would take him to the scratching post and show him what to do with his paws, actually go through the motion.

Hope this helps.