Translations

Ánh trăng mỏng quá không che nỗi
Những vẻ xanh xao của mặt hồ
Những nét buồn buồn tơ liễu rũ
Những lời năn nỉ của hư vô.

The moonlight is too faint to cover
The lake's pallidness
The willow's hanging sorrows
The cries of emptiness 


The moonlight was too faint to lighten
The pale frailty of the lake
The drooping willow in melancholy
The pleading echoes from emptiness
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That's a tough challenge. Poetry goes with culture, translating it from one language to another will filter out the uniqueness of that culture, rendering the poem into nothing.

To me I think he meant the softness moon light using the word thin. The meaning of softness here is used in its opposite sense, meaning too "bright" so all the lake, willow branches, can not be hidden.

Here is my translation, don't laugh.

Soft, feeble moon light 
too weak even to hide
paleness of gloomy lake,
in misery hang willow branches,
from emptiness, its silent wail.