Learn Finnish: Mies ei ymmärrä uusia kylttejä. - The man does not understand the new signs.

The main reason is that this is a negative sentence (ei ymmärrä). In Finnish, negative sentences always require the object to be in the partitive case. Because the man does not understand multiple signs, we use the partitive plural.

Why is the partitive plural of uusi just uusia? Isn't the partitive singular uutta?

Yes, the partitive singular is uutta. However, to form the plural, we use the plural stem. For words ending in -si like uusi, the plural stem reverts to ending in -si-. We simply add the partitive ending -a, resulting in uusia.

How exactly do we get the form kylttejä from the basic word kyltti?

Kyltti is a word ending in -i. To make the plural stem, this final -i changes to an -e-, and then we add the plural marker -i-. Because we are adding the partitive ending -ä, the plural -i- is squeezed between two vowels and turns into a -j-. So the steps are: kyltti -> kyltte- + i + ä -> kylttejä.

Why is the verb written as ymmärrä instead of ymmärtää?