unreif
unripe
Describes fruit, vegetables, or other plants that are not yet ripe and therefore not yet suitable for eating or harvesting.
Die Tomaten sind noch grün und unreif.
The tomatoes are still green and unripe.

unripe / immature
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unripe
Describes fruit, vegetables, or other plants that are not yet ripe and therefore not yet suitable for eating or harvesting.
Die Tomaten sind noch grün und unreif.
The tomatoes are still green and unripe.

The word consists of the prefix 'un-' and the adjective 'reif' (ripe/mature). The prefix 'un-' means 'not'. So 'unreif' literally means 'not ripe' or 'not mature' and is used both for fruit and for people.
Diese Mangos sind noch hart und unreif.
These mangoes are still hard and unripe.
You can recognize whether the biological or character-related meaning is intended from the noun. If people are talking about fruit, vegetables, or fruits in general, it is about the degree of ripeness. If the word refers to a person or their behavior, it means emotional or mental maturity.
Die Äpfel sind noch unreif und sauer.
The apples are still unripe and sour.
In connection with plants, 'unreif' (unripe) is often combined with specific types. Typical phrases are 'unreifes Obst' (unripe fruit), 'unreife Früchte' (unripe fruits), or 'unreife Beeren' (unripe berries).
Man sollte unreife Holunderbeeren nicht essen.
You should not eat unripe elderberries.
When it comes to people, 'unreif' (immature) often describes abstract concepts. Common combinations are 'unreifes Verhalten' (immature behavior), 'ein unreifer Charakter' (an immature character), or the statement 'er ist emotional unreif' (he is emotionally immature).
Für sein Alter zeigt er manchmal ein sehr unreifes Verhalten.
For his age, he sometimes shows very immature behavior.
green
With fruit and vegetables, 'grün' and 'unreif' can be very similar: a green banana or a green tomato is often not ripe yet. 'unreif' says this directly and precisely. 'grün' actually describes the color, but in this context it can mean the unripe state. That is why the words can easily be confused. With people, there is also some overlap, but the difference is important: 'grün' means more like inexperienced or new to a field, while 'unreif' means more like childish or not grown-up behavior.

Diese Birnen schmecken noch hart und sauer, sie sind unreif.
These pears still taste hard and sour; they are unripe.
Die Tomaten bleiben noch ein paar Tage liegen, weil sie noch grün sind.
The tomatoes will be left for a few more days because they are still green.