YOU SAID:

I shall have been there on Monday, I swear

INTO JAPANESE

私は月曜日にそこにいたはずだ、私は誓う

BACK INTO ENGLISH

I should have been there on Monday, I swear

INTO JAPANESE

私は月曜日にそこにいたはずだった、私は誓う

BACK INTO ENGLISH

I was supposed to be there on Monday, I swear

INTO JAPANESE

私は月曜日にそこにいるはずだった、私は誓う

BACK INTO ENGLISH

I was supposed to be there on Monday, I swear