Question: Why can you say “slow down” and “sit down” but not “speed down”?
Answer: You can’t down a noun!

Do you know when to use the, a, & an?
Use “the” before a noun when…

the listener knows the exact thing you are talking about “The speech was really long.” (The listener knows the speech you are talking about.) “The car that drove by was speeding.” (The listener saw the car.)
there is only one in the [...]