"Non credo che abbiano capito."

Translation: I do not believe that they have understood.

[HelenCarlsson]

Shouldn't "I don't think..." work as well?

[desifromitaly]

"Believe" è la traduzione maggiormente fedele di "credo", perché "think" corrisponde a "penso". Ma credo/penso anch'io che si possa tradurre anche con "I do not think ...". Non cambia molto :-)

[SporadicAspirant]

"I do not think that they have understood." is an accepted answer (it might not have been 8 months ago when you wrote your comment).

(I've had so many exercises, particularly the multiple choice sort, where credo was translated to I think that I had to try it here. It's possible the system didn't automatically accept the contraction don't, although I thought it was meant to.)

[Casey_09]

I did it right and they calculated it as wrong answer