Pine trees stay green through winter because their needles are built to survive conditions that would be much harder on broad leaves.
The tree does lose needles.
It just does not lose nearly all of them at once.
Needles reduce water loss
Pine needles have a small surface area compared with broad leaves.
They also have a thick waxy coating.
Both features help limit water loss.
That matters especially in winter.
Frozen ground creates a water problem
A tree may be surrounded by snow and still have difficulty replacing water.
If soil water is frozen, roots cannot take it up as easily.
Leaves that lose large amounts of water would create a serious problem.
Needles are better suited to conserving it.
The cells can tolerate cold
The recorded script also notes that pine needles contain substances that help their cells tolerate low temperatures.
The needles are not simply ordinary leaves that happen to be skinny.
They are adapted to winter conditions.
Do evergreen trees keep the same needles forever?
No.
Older needles eventually fall.
Many evergreens keep needles for several years and replace them gradually.
That is very different from a deciduous tree dropping most of its leaves in one season.
How is this different from fall color?
In many deciduous trees, chlorophyll breaks down as the tree prepares to shed leaves.
That reveals other pigments and helps create the colors discussed in why leaves change color in the fall.
Evergreens follow a different strategy.
Does evergreen mean every needle stays green?
No.
Individual needles can yellow, brown, and fall.
The tree still looks green because enough living needles remain throughout the year.
So evergreen does not mean the tree never loses leaves.
It means enough needles stay on the tree that winter never makes it look bare.
And that’s the short explanation.
Related explanations: Why do leaves change color in the fall?, Why is grass green?, and Why do sunflowers face the sun?