他很慢地走: why there is 地 if the adjective is monosyllabic?
I can't use 地 after the adjective if the adjective is monosyllabic


This is not a grammatical rule. This is an idiomatic rule and mostly for prosody concerns. 

All the following expressions are grammatically correct. Only some sounds more natural than the others:


他慢地走 &lt;- sounds unnatural
他很慢地走 &lt;- sounds fine but not idiomatic
他慢慢地走 &lt;- sounds fine and idiomatic