Board Thread:Lore Discussion/@comment-768817-20130817024153/@comment-16047389-20130920163225

They can learn the thu'um by reading it, but the Dragonborn needs dragon souls to tap into the understanding, unless another individual who already understands a thu'um willingly shares it like the Greybeards do. And they way they do it looks similar to the effects of absorbing a dragon soul.