How can autonomous private spending change budget surplus? I was reading Keynesian economics in R.Dornbusch's book where he mentioned the defects of using budget surplus as a measure where he says it can be changed and effected by autonomous private spending which I couldn't logically get.Can someone explain?
In a very rudimentary sense, the question can be answered in the following way :
We see that 
$BS = tY - G - TR $

As you put in the quote, Dornbusch is arguing that budget deficit/ surplus is not a good measure to gauge government policy. 

EXPLANATION: 
For example, a deficit doesn't always mean that $G$ is large or that the government is spending to increase income. An increase in budget deficit can occur if there is a reduction in private spending (which is $I$) - since that leads to a reduction in $Y$, which reduces $tY$ (tax collections) that increasing deficits. 
When he says that private spending can affect budget surplus, he means that private spending $I$ can affect the level of income $Y$ which can affect $tY$ (tax collections) which can impact Budget Deficit/Surplus. 
Hope this helps!