It depends on the nature of the grants. Grants that are capital in nature such as SME Go Digital is not taxable while grants received that will reduce the operating cost of the company will be taxable such as Goverment Paid Leave, Special Credit Scheme, Wage Credit, Skill Future Enterprise Grant etc. 

IRAS has made an exception to Jobs Support Scheme grant where IRAS has categorised the grant as non-taxable grant.

Generally, you must register for GST if your taxable turnover at the end of any calendar year on or after 1 Jan 2019 is more than S$1million or if at any time, you are expecting a taxable turnover in the next 12 months to be more than S$1million. 

Generally, when your non-Singapore Citizen employee (i.e. foreign or Singapore Permanent Resident employee) ceases employment with you in Singapore, goes on an overseas posting or plans to leave Singapore for more than three months, you are required to seek tax clearance for him. As an employer, you have the responsibility to file the Form IR21 and withhold all monies due to the employee for tax clearance purpose. This applies to all work pass holders including Personalised Employment Pass (PEP) holders.

From YA 2021 (Calendar Year Ending 31 Dec 2020), AIS is compulsory for employers with 6 or more employees. 

Annual deadline for AIS submission is 1 Mar every year.