Learn Finnish: Tummennammeko sävyä vähän? - Shall we darken the shade a little?

The base adjective is tumma (dark). By adding the transitivizing suffix -ntaa, we get tummentaa, meaning to make something dark. Notice that the final vowel of tumma changes to 'e' before the suffix, which is a common pattern for two-syllable adjectives ending in 'a' or 'ä'.

Why does the 'nt' in tummentaa change to 'nn' in this sentence?

This is standard consonant gradation (nt to nn). Adding the personal ending -mme creates a closed syllable, which triggers the weak grade of the consonant. So, tummentaa becomes tummennamme, and then the question particle -ko is attached to the very end.

Why is the object sävyä in the partitive case?

The action of darkening a shade is treated as a gradual or incomplete process here, especially since it is modified by vähän (a little). Because the action does not have a definitive endpoint or total completion in this context, the object takes the partitive case.

What is the difference between tummentaa and pimentää if both mean to darken?

They come from different types of dark. Tummentaa comes from tumma (dark in color) and is used for shades, pigments, or glass. Pimentää comes from pimeä (darkness from lack of light) and is used for blocking out light, like blacking out a room.