I generally have a problem with it the statement that Esperanto is poorly designed. When considering that it does enough things right. That it uses internationalisms, that it can be sung, that it gives enough expressivity, that it's mechanical enough to be learnt by it's grammar, etc.
It always sounds as if Esperanto is Latin with a thousand of exceptions, designed like french with spoken language does not equal the written text of the language, etc.
When in fact the opposite is the case. People then point to one of the current language projects, which are supposedly "better" in one dimension or another. That's just optimizing to some standard of perfect.
That sounds interesting. Esperanto has no noun-declinations, it's an agglutinating language, you don't bend words (= declination).
But what is barely resembling that what you mention is the two cases of the language, which is nominative and the so called "accusative". Which is adding -n to words to make them an object, depending on whether the verb of the sentence needs one or not. This case also is not just for objects, but also for directions, for measurements and time. That combination normally confuses the heck out of people.
Which is why there is also an in-joke in the Esperanto community "don't forget the accusative", because people forget it or apply it too often.