Hallo Vidya-manuS und sfpugh,
I agree with sfpugh, both "Ich kenne dich, du machst immer Witze." and "Ich kenne dich, du erzählst immer Witze." are fine to use.
In regards to "Magst du das Denkmal sehen?" - "Would you like to see the monument?", you are absolutely right das Denkmal needs to be in the accusative case, however the article das doesn't change hence it is already accusative.
Also just as a side note, instead of saying "Ich habe einen Zweifel." you can just say "Ich habe Zweifel." -"I have doubts." as that would be the more common expression.
Bis zum nächsten Mal!