"Gå långsammare!"

Translation: Walk more slowly!

[podgorsk]

Why GO is not accepted?

[devalanteriel]

I've added that. :)

[HelenCarlsson]

should be a short "a" in "långsammare": http://sv.forvo.com/search-sv/l%C3%A5ngsammare/

[jerrethMobil]

Walk slower isn't accepted?

[HelenCarlsson]

Slower is an adjective, right? And I guess you need an adverb here.

[podgorsk]

It depends which version of English one uses, to quote:
adverb
at a slow pace; slowly: the train went slower and slower | [in combination] : a slow-moving river.

[devalanteriel]

I've added that. :)