Some issues of nurbs

I did some research in nurbs when I was modeling ,I followed the tutorials sometimes but it didn't work in some modeling.

1:booleans

I need a lot of booleans in my model,but sometimes when i did this ,
that yellow warning appeared just like : //Warning: Operation has history, inputs are not deleted

but i've deleted all the history and also freeze the transiformation etc.

what am i doing wrong? I've got a feeling it's really simple! Maybe some mistakes of my models lead this issue?I really can't figure it out.


2：triming
After I did some trim successfully,there comes problems!   I couldn't selet the object that i want to trim when i click the trim button  ,nothing happened, i confirmed that i 've tick the selection of face also, I 've asked some classmates but still couldn't solve it.


3.Circular fillet

I could use it somewhere ,but when i was unluck ,it went wrong , I hit this button and regulated the data to less than 1 ,but nothing happed ,or just fillet the surface inside, really strange,but I think this maybe the shapes of models related```


I 'll find more tutorials to solve these issues,otherwise , maybe ask some professinal persons for help.