yeah, that decription ingame isn't great..
he gives you the three words to summon him before you leave the Soul Cairin.
Use three dragon souls so you can use all three words, then when you use it to summon him in Skyrim, he will teach you a word to the new shout. the quest will update, summon him again to get the next word
no, you should have the shout "Summon Durnehviir" in your shouts menu. Unlock that shout with dragon souls, then summon Durnehviir anywhere in Tamriel, he will then teach you the Soul Tear shout's first word. Summon him again to learn the second word, then summon him once more to learn the third word
There are a couple of Shouts that are like that.
At least you will know this in the future.
atleast you figured it out
better late than never, right