The first two sentences should be read as "flowers-non-flowers, fog-non-fog", which gives people an unpredictable feeling first. Both "non-flower" and "non-fog" are negative, but they contain a self-evident premise: like flowers, like fog.
Therefore, it can be said that these are two clever metaphors. Express a kind of nostalgia and regret for the beautiful people and things that have existed in life and disappeared.
The whole poem consists of a series of metaphors, which are obscure and true in description, with rhythmic put in order and intricate beauty in obscurity. It is a masterpiece of love poems. Later generations used to compose music, which was widely circulated.
Extended information
Flowers are not flowers
Tang Dynasty: Bai Juyi
Flowers are not flowers, fog is not fog, it comes at midnight and goes at dawn.
how long does it last? It was like the clouds in the morning when they left.
Translation: Like flowers instead of flowers, like fog instead of fog. Arrive at midnight and leave after dawn. How much time can you have when you come like a spring dream? When I went there, it seemed as if the clouds were gone and there was no place to find it.
Creation background
There are two poems in Bai's Collection of Changqing, namely, Tomb of the Mother and Poems of Jane, both of which are mourning works. This poem "Flowers Are Not Flowers" is the same volume as the above two poems, and it is compiled later. The poem "Flowers Are Not Flowers" was written for the same purpose at about the same time as "Poems on Bamboo Slips". According to poetry, the past is beautiful, but it is like a dream, and there is no longer a sigh.
Baidu encyclopedia-flowers are not flowers.