In Dr. Mastronarde’s Introduction to Attic Greek, Unit 24, Part II, exercise 19 on page 203, I am supposed to write, “They made an agreement.” I wrote, “συνέβησαν,” while the answer book wrote, “συνἔθεντο”
Is there a difference? Just one active and one passive?
This is the second time you have described a verb as passive when in fact it is middle. You should revise the difference and make sure you understand what “middle” means. look at unit 11.
Its very clear that M. wants you to use συντίθημι which was introduced in the previous chapter because you are practising μι verbs. I can see that you saw συμβαίνω on p 201 and naturally thought that’s what you should use. But if you look in the glossary p 468 you will see “agreement: make an agreement, συντίθεμαι (23)”.
You should approach these exercises with the question what am I supposed to be practising rather than what is possible or what is the “right” answer. There are many ways of expressing the same idea in Greek but the context may mean that one word rather than another is more appropriate. Dont get the idea that words with similar meanings are interchangeable. They might be but on the other hand might not.
Shorn of any context either verb would work but that’s not the point of the exercise.
Look at LSJ here
http://www.perseus.tufts.edu/hopper/morph?l=συνέβησαν&la=greek#lexicon
and here
http://www.perseus.tufts.edu/hopper/morph?l=συνἔθεντο&la=greek#lexicon
to see the nuances.