He doesn't learn by level. First, go underground (anywhere) and find a Heart Scale. Now go to Pastoria City and by the city map sign, go down 7 paces, and then by the house on the left, go to the right house (8 steps) and talk to the guy there. (He will be saying 'Here where I am standing, I'm smack-dab in the middle of town.'). go in. Talk to the other guy and give him your Heart Scale. He'll ask you what Pokemon should learn what move and your wanted move will be there but you have to have an empty space for it and if you don't have an empty spot for it then he'll ask you to delete 1 of your moves however he can't delete HM moves. Also if you're replacing for a HM, go to Canalave City and the house below the PokéMart and talk to the guy in the house and tell him to forget the HM you want gone then you can return to Pastoria.
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On a normal level-up scale, Torterra learns Wood hammer at lvl 1 (!) so therefore you need to bring it to a move tutor with a Heat Scale to relearn the move.
at lv one . check marriland and serebii or bulbapedia
After torterra is able to learn it, if you select no then you can use a heart scale and then teach it the move using the heart scale tutor
You have to upgrade your torterra to level 39 before it can learn synthesis.
Torterra can learn Leaf Storm at level 57.