"He is a man."
Translation:On jest mężczyzną.
Well, as you probably know, Polish has 7 cases, which depend on the function in the sentence, the fact that certain verbs or prepositions need certain cases, etc. Without understanding them, you will not be able to learn Polish.
Now, Instrumental is a case that has three main meanings: one, that you use X as an Instrument; two, that you are doing sth "with X"; and three, in defining. At the beginning of the course, we focus on the third meaning.
First, I'd recommend taking a look here: https://www.duolingo.com/comment/16373167 - especially Parts 1 and 2.
Secondly, you can search for topics about cases in this post: https://www.duolingo.com/comment/16296174
As for the -ą ending, it is true for nouns that end with -a in their basic, Nominative form. These are almost all feminine nouns and a few masculine ones. As "mężczyzna" means "a man", it is one of those masculine examples.
After forms of the verb "być", object nouns are always in the Instrumental.
For more details, please see: https://forum.duolingo.com/comment/16373167/A-guide-on-X-is-Y-and-This-is-Y-constructions as mentioned above....
That said, since it does make a big difference in Polish, you can use the onscreen buttons in the web interface, and lots of mobile keyboards allow accented characters. That doesn't really suffice for the timed tests, but your operating system may have a Polish mode available for English keyboards (certainly my Windows 8 version does).