Is there any difference between "the fly is small" and "the fly is little"?
Not in meaning. To my ear, "little" sounds a bit more childish than the very objective-sounding "small". There are certain situations where it's more or less idiomatic to use one over the other, though.
I do not know where to report this problem I cannot proceed any further after this lesson. the program seems to have frozen
Fliege should be capitalised -- it's a noun.
The noun Fliege has feminine gender, that is why it's die Fliege in this sentence with the feminine (nominative singular) article die in front of it.
die is also used in the plural, true, but here it's the feminine article.
Earlier duo says klein means quiet. Now it says small... am I missing something?
Quiet is leise
Small is klein
Both words have -lei- in them -- is that perhaps why you are confusing them?
wait wait wait, "Fliege" is feminine, hence "Die Fliege". So why "ist klein" instead of "ist kleine" ? You know, like "Eine kleine Fliege"... ?
Because predicate adjectives (after the verb "to be") do not have any endings for gender/number/case in German.
die Fliege ist klein, der Hund ist klein, das Pferd ist klein, die Tiere sind klein.
Unlike, say, in French or Greek.