I can't use 地 after the adjective if the adjective is monosyllabic (so 他慢地走 isn't acceptable). Then why in this sentence is there 地 after a monosyllabic adjective?
Is it because I can use 地 if there is 很 before the adjective ?
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:
Darlin' who is teaching you this stuff? I hope you are not paying too much!
You can say: 她很慢地走。or 她慢慢地走。
Better might be: 她在很慢地走。 or 她走得很慢。
Goodnight, Miss Chestnut!