The non - conversion of account into NRE account is violation of FEMA guideline and attract penalty for violation of same on adjudication.
2. There is no tax as you have already paid tax when you were in Singapore.
3. This will attract penalty as it is your saving account in India tax won't be there as it is not violation on Income tax department but fema/ rbi guidelines.
There is no specific penalty provided for not converting an ordinary savings account into a non-resident ordinary account. However, Section 13 of the Foreign Exchange Management Act, 1999 provides that if any person contravenes any provision of this Act, or contravenes any rule, regulation, notification, direction or order issued in exercise of the powers under this Act, or contravenes any condition subject to which an authorisation is issued by the RBI, he shall, upon adjudication, be liable to a penalty up to thrice the sum involved in such contravention where such amount is quantifiable, or up to Rs 2 lakh where the amount is not quantifiable. Where such contravention is a continuing one, further penalty that may extend to Rs 5,000 for every day after the first day during which the contravention continues. It is, therefore, desirable that an account should be converted into an NRO account without delay.