YOU SAID:
my youngest son has stolen my biscuit.
INTO JAPANESE
私の末っ子が私のビスケットを盗んだ。
BACK INTO ENGLISH
My youngest stole my biscuits.
INTO JAPANESE
私の末っ子は私のビスケットを盗んだ。
BACK INTO ENGLISH
My youngest stole my biscuits.
You've done this before, haven't you.