1.opportunity is like sink cost It never come back again
2. He's very Civill, he respects everyone even though he loves people who is ignored by public
3. You must be honest when you have trade with the other people
4. Real Gentle man is humble (modest), polite to everyone and polite
5. No mercy means not tolerant, not merciful 聼起來自然嗎?

1) I'm a bit confused as to what you were going for here, would you mind trying to explain?
2)He's very civil and respects everyone. Even the people who are ignored by the public.(You had that one for the most part :)
3)For the most part, this is correct. Just drop the have as you don't posses trade. Drop the "the" as well.
"You must be honest when you trade with other people."
4)Real gentlemen are humble, modest(Humbleness and modesty have differences, so no parenthesis are required.), and polite to everyone.(No need to mention politeness twice.)
5)"To lack mercy is to lack tolerance / To lack mercy is to lack tolerance, not mercy."