I would second what Jellei says here. As you can see in a lot of the vocabulary you have already gotten, the i softens, or palatalizes, the consonant that precedes it, which in Polish turns the s into the same sound as sz (sh in English). It seems that, when the i is already making the consonant soft, Polish does not use the special softened letters (sz, cz, ń, ź and there are others I am sure I am not thinking of). It also changes the ł, which clearly descends from an unpalatalized l, into the regular, palatalized l. Of course, I am far from an expert in Polish, so do take everything I have said here with a grain of salt.
I hate that the Duolingo font makes them almost identical...
Well, Ż (with a dot) makes a sound that in English is roughly represented by ZH. Although of course it's not common in English.
Ź (with an 'accent'), like other consonants written with an accent, is a patalalized version of Z. Those take some time for a non-Slavic person to learn.