Learn Hardcore Dutch: Vergeet de deadline niet.

Because vergeet is in the imperative mood, which by default addresses the second person jij (you). In Dutch imperatives you drop the subject pronoun, so Vergeet de deadline niet literally means “Don’t forget the deadline,” with the subject “you” understood but not spoken.

Why does the verb vergeet come first?

In Dutch imperatives the finite verb always comes first, followed by the object or other elements. That’s why you see Vergeet at the start, rather than the normal subject–verb word order you’d use in a statement.

What does niet modify here, and why is it placed at the end?

Niet negates the action of the verb phrase. When negating an imperative that has a direct object (here de deadline), niet usually comes after that object. So the basic order is: [Imperative verb] + [object] + [niet].

Why is it de deadline and not het deadline?

Deadline is a borrowed English word that is treated as a common gender noun in Dutch, so it takes the definite article de. Most loanwords ending in a consonant and referring to things or events default to common gender.

Could you use the plural deadlines instead?