Analysis:

This quatrain by Saadi Shirazi expresses a deep sense of betrayal and heartbreak. The speaker is lamenting the change in their beloved, once seen as angelic and comforting, now perceived as evil and cold. Let's break down each line:

Line 1: آن ماه که گفتی ملک رحمانست

"That moon you claimed was God's angel,"
The beloved was once idealized, compared to a beautiful moon, a symbol of divine beauty and purity.

Line 2: این بار اگرش نگه کنی شیطانست

"If you look at it now, it’s Satan."
A stark contrast is drawn, as the beloved is now seen as the embodiment of evil, comparing them to Satan. This suggests a significant change in the beloved's behavior or character.

Line 3: رویی که چو آتش به زمستان خوش بود

"That face, as warm as fire in winter,"
The beloved's face was once seen as a source of warmth and comfort, like a fire on a cold winter's day.

Line 4: امروز چو پوستین به تابستانست

"Is now as cold as a winter coat in summer."
The image of a winter coat in summer conveys a sense of discomfort and inappropriateness. The beloved's affection, once comforting, is now perceived as burdensome.

Themes explored in the quatrain:

Betrayal and heartbreak: The speaker is clearly experiencing deep emotional pain caused by the betrayal of their beloved.
The transience of love: The quatrain suggests that love can be fleeting and that even the most beautiful and passionate love can turn sour.
The duality of human nature: The beloved is depicted as having both angelic and demonic qualities, highlighting the complexity of human nature.

This quatrain is a poignant expression of heartbreak and disappointment. It captures the universal experience of feeling betrayed by someone you love.