klein = positive form, kleiner = comparative form. So, "Der Mann ist kleiner" means "The man is smaller".
However, in this case, the adjective needs inflecting, as it is in front of the noun. So "Das ist ein kleinER Mann" = "That is a small man", as klein must be inflected by adding -er in this case (mixed inflection, masc. nominative). If you want the comparative form, kleiner also needs inflecting, so "Das ist ein kleinerER Mann" = "That is a smaller man".
You have to go to other sites for that. That's just how Duolingo works. You're welcome to read mine, if you want.
No, you need to decline the adjective.
indefinite article -> mixed inflection
The "er" in "kleiner" is supposed to be pronounced as a vowel. It would be wrong to pronounce it as a consonant.
Two things you should pay attention to are both, the "ein-words" and the actual noun itself.
Firstly, "ein" was said, "kleine" should not follow (it would have been "eine" instead).
Next, "Mann" is a masculine noun, so an -e ending is only ever used if "der" proceeds the adjective for masculine nouns. An Example: "Der kleine Mann ist klein."
So when you hear: "...ein....adjective.....masculine noun", you should instantly know that that adjective needs an 'er' ending. Eventually you will hear a difference! :)
das is the article for words of neuter gender.
The word Mann has masculine gender, so das Mann is wrong -- it has to be der Mann.
klein is wrong here because attributive adjectives (basically, ones used before a noun) need an ending that depends on gender, number, and case as well as on what sort of word (if anything) is in front of the adjective.
After a definite article, adjectives take weak inflection, which is -e in the nominative singular for all three genders, so it is der kleine Mann, die kleine Frau, das kleine Kind.
Perhaps you were led astray by the fact that it's ein Mann, eine Frau, ein Kind, but the indefinite article ein, eine, ein does not inflect quite like an adjective.