Tom loopt de bibliotheek binnen.

Tom walks into the library.

Breakdown of Tom loopt de bibliotheek binnen.

• Tom: Tom
• de bibliotheek: the library
• binnenlopen: to walk into

Questions & Answers about Tom loopt de bibliotheek binnen.

Why is it loopt and not gaat in this sentence?

Dutch often prefers a manner-of-motion verb where English might use a more general verb like “go”.

• lopen = to walk (focus on the way of moving, on foot).
• gaan = to go (more general, can be any way of moving).

Tom loopt de bibliotheek binnen. focuses on the fact that Tom is walking into the library.
You could say Tom gaat de bibliotheek binnen, but it would sound less natural and slightly odd in many contexts. Native speakers almost always choose lopen, rennen, fietsen, rijden, etc., when they know the manner of movement.

What exactly is binnen doing here? Is it a separate word or part of the verb?

Here binnen is the separable particle of the separable verb binnenlopen.

• Base verb (infinitive): binnenlopen = to walk in / to walk into.
• In a main clause, separable verbs split:
  • Tom loopt de bibliotheek binnen.
    
    • loopt = finite verb (present, 3rd person singular)
    • binnen = particle, moved to the end

So grammatically, you can think of the whole predicate as:

Tom binnenloopt de bibliotheek. → surface form: Tom loopt de bibliotheek binnen.