Hitotsu mo ame ha nagasanai
 

What does that mean?

"Not a single drop of rain falls."

Why flows-not instead of one-not?

Instead of not-one sees look at it with a negative verb.
"it doesn't even fall a single drop of rain".

Verbs! My nemesis, I should have known!

Thank you. It makes sense. :)

Bullets—my only weakness! How did you know?

Way off.

No, sir.
______

Since no one is getting it correct, I shall deal with this romaji question.

Just look at the transitive verb　流さない everyone. This is about rain washing something away, not rain drops themselves falling. Besides, you never count "rain" ひとつ, ふたつ、 etc.

"Rain doesn't wash away even one of those."

What "those" are, only the OP would know. It should have appeared before this sentence.

Yes, and a familar noun would be 流し, right? :) This vocabulary is stuck to my kitchen sink (on a post-it) LOL
Ah, is the past form of 流す 流した?

yes
filler space filler space

Cryptic answer:

One even under rain flows not. Hrm-hmm!

Also, my dict says "following a number, mo indicates the speaker considers the number inordinately large." Being a person who finds the number 8 dauntingly large I can appreciate how much bigger one is over zero, demo my malnourished 分る is mada 危ない橋を渡る. :D