"have down" is not a grammatically correct phrase in English.
If you are looking for a phrase to express understanding or mastery, you can use "have down pat" or "have nailed down". Example: After months of studying, I finally have the material down pat.
Exact(53)
They don't have down payments".
I have Down Syndrome.' He's funny too".
The strong points, they have down pretty much already.
What do you think I have down there?
Many fetuses found to have Down syndrome are aborted.
"He's one of the best players we have down there.
As a catcher, you never have down time.
Similar(4)
"We don't have down-time with these servers," Jones says.
I decide to seek out other wildlife (even business travellers have down-time).
Food manufacturers and textile firms have down-at-heel outposts here.
They have down-to-earth decency, a rugged decorum, easy-going diligence.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com