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.
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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.
Torterra can be taught Solar Beam with TM22