Analysis:

This quatrain by Saadi Shirazi beautifully captures the idea that true pleasure and happiness are found in the company of a beloved. Let's break down each line:

Line 1: گویند هوای فصل آذار خوشست

"They say the air of the month of Azar is pleasant,"
Here, Saadi refers to the month of Azar, which corresponds roughly to November in the Gregorian calendar. Traditionally, Azar is considered a pleasant month with mild weather.

Line 2: بوی گل و بانگ مرغ گلزار خوشست

"The scent of flowers and the song of the garden bird are delightful,"
These lines describe the sensory pleasures of the natural world: the sweet fragrance of flowers and the melodious songs of birds.

Line 3: ابریشم زیر و نالهٔ زار خوشست

"Silk beneath and a mournful melody are enjoyable,"
This line might seem contradictory at first glance. Silk is associated with luxury and comfort, while a mournful melody is typically associated with sadness. However, in the context of love poetry, a mournful melody can be seen as a way of expressing deep emotion.

Line 4: ای بیخبران اینهمه با یار خوشست

"O unaware ones, all this is pleasant only with a beloved."
This line is the crux of the quatrain. Saadi argues that while the pleasures of the natural world and material comforts can be enjoyable, the ultimate source of happiness is the companionship of a beloved. All the beauty and pleasure in the world is enhanced and made truly meaningful when shared with a loved one.

Themes explored in the quatrain:

The importance of love: Love is presented as the ultimate source of happiness and fulfillment.
The interconnectedness of pleasure: The quatrain suggests that all pleasures are interconnected and that the enjoyment of one pleasure enhances the enjoyment of others.
The role of the senses in experiencing love: Saadi uses imagery related to the senses (smell, sound, touch) to convey the depth of his feelings.

This quatrain is a celebration of love and the joy it brings. It suggests that while material pleasures and natural beauty can be enjoyable, the true meaning of life is found in human connection and the love we share with others.