4a. John has become a rather good actor.
4b. John has became a rather good actor.( I chose 4a)
5a. Some athletes have taken drug test to prove they are not on steroids.
5b. Some athletes have took drug tests to prove they are not on steroids
( I chose 5a)
7a. I was interested in working on the presidential campaign if the opportunity still exists.
7b. I am interested in working on the presidential campaign if the opportunicty still exists.
4a is correct.
Neither 5a nor 5b is correct. Here's 5a in corrected form: Some athletes have taken drug tests to prove they are not on steroids.
What do you think about 7a and 7b?