Learn Dutch: De lege bus vertrekt nu. - The empty bus departs now.

Because it comes directly before the noun bus. In Dutch, when an adjective is placed before a noun, it usually gets an -e added to the end. Since bus is a de word, the adjective takes this -e ending.

If the dictionary word for empty is leeg, why doesn't lege have two e's?

This is because of a Dutch spelling rule. When we add the -e ending to leeg, the word splits into two syllables: le-ge. Because the first syllable ends in a vowel, that vowel automatically gets a long sound. Writing two e's to make leege isn't necessary and breaks Dutch spelling rules.

What is the dictionary form of vertrekt?

The dictionary form (infinitive) is vertrekken, which means to leave or to depart. Here we use the form vertrekt because the subject is de lege bus (which acts as an 'it'), so we use the stem of the verb plus a -t.

Could I put nu at the beginning of the sentence?