There're 4 types of items that you and I should declare when entering
Singapore:
(1) Prohibited items
(2) Controlled or restricted items
(3) Taxable or dutiable items exceeding your GST Relief or duty-free
concession
(4) Items for which no GST Relief or duty-free concession is granted
If you're not sure whether your medicine is allowed, or whether the plant you brought is endangered, or whether the gift for your singaporean friend needs a permit, or whether your kid's toy is considered danger, or whether you have to pay duty for the cigerettes in your pocket, then just ask Customs at "Red Channel, Goods to Declare". Don't go to "Green Channel, Nothing To Declare"! 'Cause if you're wrong, you might get a fine up to $5,000 for making an incorrect declaration .
For more information, they prepared "Customs Stations" at Changi, or you can visit their website.