1.if I define iSlippage =2, is it equal 20point under 5 - digits system?
2.How can I know the slippage after a order placed finished?
1. No, it equal 2 point under 3 or 5 digits system, and it will be equal 2 pips (20 point) on 2 or 4 digits system.
2. unfortunately there is no function in mt4 by default that may it be obtained instantly.
we should check it out by writing some code corresponding needs, for example can be found here: http://www.forexpeacearmy.com/community/threads/mql-code-to-record-order-execution-duration-and-slippage.18985/
p.s. some brokers have to provide facilities such as notes slippage in the Orders Comment (I forgot the broker name). But it would be better if you note itself.