I have a dachshund dog (HOT DOG ) and he has hair loss all time, I want to know why and what can I do? Because before never had this problem.

Have his thyroid levels checked.

Could be quite a few things unfortunately.
Could be thyroid problems like KISS said, could be mites, allergies, skin problems, excessive scratching, contact dermatitis.

You really need to talk to your vet to find out the exact reason.

So is he just shedding a lot, or actually losing clumps of hair? The shedding could be seasonal, but if it's actual clumps of hair, then it could definitely be something more serious as KISS and Shaz mentioned. One thing you can look at right away is a food allergy. What are you feeding him now?

Hair loss how? Is he shedding because we're heading into spring? Is he going bald in places? Has their been any stress in his life? Please be more specific :)