Learn Dutch: Hij wilde er gisteren snel overheen springen. - He wanted to jump over it quickly yesterday.

In Dutch, indicating movement completely over an obstacle often uses the two-part preposition (circumposition) over ... heen to emphasize the full trajectory from one side to the other. When we replace the object with er (meaning "it"), the phrase becomes er ... overheen. Using just erover is grammatically possible but less dynamic, and it is more commonly used to mean "about it" (like talking about a topic).

Why is er ... overheen split up by the words gisteren snel?

Combinations of er and a preposition naturally split up in Dutch sentences. The weak pronoun er gravitates to the beginning of the middle section of the sentence, landing right after the finite verb wilde. The prepositional part, overheen, is pushed toward the end of the clause, just before the final verb springen. This creates a frame where time (gisteren) and manner (snel) easily fit inside.

Could I just keep the word together and say "Hij wilde eroverheen gisteren snel springen"?

No, that would sound unnatural to a native speaker. While shorter pronominal adverbs can sometimes stay together in specific contexts, the presence of time (gisteren) and manner (snel) almost always forces the word er to split from its preposition. The time and manner details must go between er and overheen.