Learn Finnish: Tämä pienikokoinen pentu on tosi hyvätapainen. - This small-sized puppy is really well-mannered.

Both follow the Modifier + Noun stem + -inen pattern. Pienikokoinen combines pieni (small) + koko (size) + -inen. Hyvätapainen combines hyvä (good) + tapa (manner or habit) + -inen. The suffix works just like the English endings in small-sized and well-mannered.

Why are the first parts of these compounds (pieni, hyvä) in their basic dictionary form?

In property compounds, when the first part is an adjective describing the noun, it usually stays in the nominative (dictionary) form. If the modifier were a noun instead, it would typically take the genitive case, for example, sudenkokoinen (wolf-sized).

How would these -inen adjectives change if the sentence required a different case?

Words ending in -inen change to an -ise- stem before taking most case endings. For example, if you bought the puppy, the object would take the genitive form: Ostin tämän pienikokoisen pennun. If you were just petting it, the partitive would be Silitän tätä pienikokoista pentua.

Tosi means true, so why is it translated as really here?