Becker pages 29 , 30 of chapter 3

Could some one explain when is a gain an ordinary gain , when 1231 gain e.t.c  .....I dont get it .

A 1231 gain is a gain on either depreciable or real property (not personal property).  It is for an asset held more than 12 months.

A 1245 gain is for machinery and equipment only.  You will never see a 1245 loss as all you can have is a gain.  This gain will just be the amount of accumulated depreciation that you write off when you sell the equipment.  The rest of the gain if it is there is a 1231 gain.

A 1250 gain is for buildings/real property only.  It is also just a gain only (can't have a 1250 loss) and regular income is the difference between straight line and accel depreciation.  The rest of the gain is 1231.