What part of speech is before in to boldy go where no man has gone before?

"Before" in the phrase "to boldly go where no man has gone before" is a preposition.

Here's why:

* Prepositions show the relationship between a noun or pronoun and another word in the sentence. In this case, "before" shows the relationship between "go" and the phrase "where no man has gone before." It tells us that going is done *before* the exploration of uncharted territory.

Let me know if you'd like to explore other parts of speech in the phrase!