The singular form of the demonstrative pronoun 'these' is this.
The singular form of "cuffs" is cuff.
The singular form of feet is foot.
The singular form of "dice" is "die."
The singular form of "oases" is "oasis."
The singular form of "wash" is "washing."
The singular form of "hypothysis" is "hypothesis."
The singular form is ombudsman.
The singular form is hilum.
The singular form is money.
it is already in its singular form.
The singular form in stimulus.