What it's the difference between zridka and casto?
zřídka - rarely, seldom
často - often
Can we also use "hodně" in stead of "velmi" here?
I believe it didn't get accepted. Next time, I'll report it.
It is accepted. You probably had some other mistake in your answer. That is why you should always report a complete sentence when some translation is rejected.
Yes, the course is just too good, I confess ;-)