This is an irregular verb. It follows the conjugation pattern of the verb parecer.
PresentPresente
Te agradezco mucho tu ayuda.
I thank you very much for your help.
PreteritePretérito indefinido
Los estudiantes agradecieron la paciencia del profesor.
The students thanked the teacher for his patience.
Present SubjunctivePresente de subjuntivo
Espero que agradezcas el regalo de tu tía.
I hope you thank your aunt for the gift.