Learn German: Eine dunkle Wolke kommt über den Berg. - A dark cloud is coming over the mountain.

The base adjective is dunkel (dark). When a German adjective ends in -el or -er (like dunkel or teuer), it usually drops the unaccented e before adding the grammatical ending to make it easier to pronounce. Because Wolke is a feminine noun in the nominative case, the adjective takes an -e ending, resulting in eine dunkle Wolke.

The translation says 'is coming', so why does the German sentence just use kommt?

German does not have a separate continuous tense like English does. The simple present tense verb kommt translates to both 'comes' and 'is coming'. You just rely on the context of the sentence to know which one makes the most sense in English.

Why is it über den Berg (accusative) instead of über dem Berg (dative)?

über is a two-way preposition. It takes the accusative case when it describes movement toward a destination or a change of location, and the dative case when describing a fixed position. Because the cloud is actively moving across the mountain, it requires the masculine accusative form den Berg.