Soft Potatoes are not always bad as long as the potato is not too soft.
If the potato is really soft and mushy then the potato is bad and should not be eaten.
When potatoes go bad they start to shrivel and then get soft and the softer they are the worse off they are.
Potatoes that are bad will also start leaking fluid and smell bad.
A good potato should be firm to the touch but a little softness is okay but overly soft potatoes should be thrown out.
If the potato has sprouts on the potato they are still good and you can just cut the sprouts off the potatoes and eat the rest of the potato.
Potatoes are best stored in a cool dark place to prevent spoilage.