After three days of using the gel, my mouth has returned to its familiar self.
( ( After ( ( three days ) ( of ( using ( the gel ) ) ) ) ) ( , ( ( my mouth ) ( ( has ( returned ( to ( its ( familiar self ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN After) (NP (NP (CD three) (NNS days)) (PP (IN of) (S (VP (VBG using) (NP (DT the) (NN gel))))))) (, ,) (NP (PRP$ my) (NN mouth)) (VP (VBZ has) (VP (VBN returned) (PP (TO to) (NP (PRP$ its) (JJ familiar) (NN self))))) (. .)))
They had just undergone oral surgery. 
( They ( ( ( had just ) ( undergone ( oral surgery ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBD had) (ADVP (RB just)) (VP (VBN undergone) (NP (JJ oral) (NN surgery)))) (. .)))