Translation:The test today was easy.
The test is already finished, it WAS easy/simple. That's why deshita takes place.
I'm obviously not blast, but deshita (でした) is past tense - the test has already happened. It WAS easy.
Same thing as mashita, and mashitaka. (ました、ましたか) as opposed to ます
What us the difference between "Yasashii" and "Kantan" with respect to tests?
I'm not entirely sure, but I think one is more like simple while the other more like easy.
But maybe I have just a fake memory because I have read simple in the comments section.
Can you give us examples, please, where they are not interchangable ? Thank you
Basically, kantan means simple, yasashii means easy. Something can be easy but not simple, that's where they are not interchangeable.
Wouldn't "Today's test was very easy." also work? Considering that 今日のテスト is used I feel it might be okay.
I'm sure you can also say "Today's test" was very easy. That should be accepted.