税是怎样计算的？

about the tax (tax rate etc), you can get the information in IRAS website

http://www.iras.gov.sg

税率的规定，我也不知道。因为知道了也对我的税率没有影响，除非我想避税。不过我的薪水少得不至于交很多的税，所以我就没有考虑税率。

保税好像是四月份截止。税单寄到IC 上面显示的地址

Your company (and pass employers you worked for in the past year) will send you a copy of IR8A for you e-file. Just copy the number on the form to the online form is OK.
You can call their hotline if you have any doubt.

税单寄到家里－－一般是IC上的地址

how if haven't pay the tax in time?

How do I update my personal particulars?

You may call our Individual Income Tax Helpline at 1800-3568300 during our office hours from 8am to 5pm from Monday to Friday; and from 8am to 1pm on Saturdays or email us. If the update is a change in name, please send or attach a copy of your NRIC, Deed Poll or Passport. You can also write to us at:
Inland Revenue Authority of Singapore
55 Newton Road
Revenue House
Singapore 307987

如果我的理解没有错的话，报税和缴税是两个不同的概念，报税是呈报你的收入之类，算出应该交多少税之后在一定时间内得去缴税。没有或者推迟报税/缴税的话，都会有麻烦

http://www.iras.gov.sg/ESVPortal/iit/iit-se-a1.1.20+penalties+for+late+or+non-filing+of+it+forms.asp
↑What are the penalties for late or non-filing of income tax form

http://www.iras.gov.sg/ESVPortal/iit/iit-enf-v.a.1+penalities+for+late+or+non+payment.asp
↑What are the penalties for late or non-payment?

所以一直没收到。

如果是，赶紧去更新地址吧。速速！