About individual tax return

1. What is the tax rate of salary tax? How to compute the amount of tax payable?

Your net chargeable income, i.e. assessable income after deductions and allowances, is charged at progressive rates. But if what you need to pay on the basis of your net chargeable income exceeds the tax charged at standard rate on your net total income, i.e. assessable income after deductions but before allowances, then you have to pay the lower amount of tax.
Assessment year refers to April 1 to March 31 of the following year. Tax payable is computed on the net chargeable income an individual from April 1 of the previous year up to March 31 of the current year. You may refer to the following link for more information on the tax rates and computation of salaries tax:
https://www.ird.gov.hk/chi/pdf/pam61c.pdf
2. When is the deadline for filing a salary tax return? What is the penalty if the deadline is not met?

You will have to file the salary tax return within a month from its issue date. If you fail to do so, the Inland Revenue Department may compute and issue your tax demand note based on an estimated net chargeable income and you may end up paying a higher amount of salary tax. You might also be prosecuted or fined for the delay in filing tax return.