YOU SAID:
jack shall sit on a tack
INTO JAPANESE
ジャックはタックの上に座る
BACK INTO ENGLISH
Jack sits on the tack.
INTO JAPANESE
ジャックはタックの上に座っている。
BACK INTO ENGLISH
Jack is sitting on the tack.
INTO JAPANESE
ジャックはタックの上に座っています。
BACK INTO ENGLISH
Jack is sitting on the tack.
That didn't even make that much sense in English.