"He likes this clock."
Translation:On lubi ten zegar.
In Polish, making a word diminutive is very common. Instead of "zegar" you might hear a native say " zegarek". Both are correct and in fact the diminutive is almost considered more formal, at least where I come from. Although, for the purpose of this course, I would suggest to sticking what Duo says to do. ^^
It CAN be about size. It CAN be formal. It is meant to meant the object appear smaller, cuter. Sometime this implies politeness and in a polite conversation using the non-diminutive form can almost be considered hostile. It is mostly about context and interpretation, as with most language.
Ah, I get now what you mean. Actually, many people think they are more friendly with diminutives, but to me it doesn't really matter. It's overuse might have an opposite effect, when you make yourself sound fake or condescending (especially when one starts using amplified diminutive). I would never use diminutives just for the sake of being friendly, but maybe it's just me.