Learn German: Ich verstehe die Schwierigkeit. - I understand the difficulty.

The suffix -keit is used to turn adjectives into nouns. A great rule to remember is that every noun ending in -keit is feminine. Because of this predictable gender, we know we must use the feminine article die.

Why does this noun use -keit instead of -heit?

When turning an adjective into a noun, words that end in -ig (like schwierig), -lich, -sam, or -bar almost always take the suffix -keit. Other adjectives usually take -heit (like die Krankheit from krank). Both suffixes work just like "-ness" or "-ity" in English.

Is die Schwierigkeit in the nominative or accusative case here?

It is in the accusative case because it is the direct object being understood. Fortunately, for feminine nouns, the accusative article die looks exactly the same as the nominative (subject) article.

If I wanted to say "I understand the difficulties," how would the noun change?