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On a particular forum is a banner reading:
此乃舊貼唯讀區
which I imagine is a highfalutin way of saying that this thread is read-only.
Now, 乃 has many senses in my dictionary. What is the sense of 乃 being used here? My best guess would be "only", in the sense that this page is "nothing more than" a read-only area.
Here "乃" means "be". This sentence means "This area is for old posts, and these posts are read-only."
Next time, if you find "此乃" these two characters together or a noun+乃 together, "乃" properly means "be". For example:
此乃我的书.
This is my book.意识乃是大脑的一种自然属性。
Consciousness is an emergent property of the brain.
此乃我的书 sounds a little funny/silly because 我的 is a modern expression. In a formal occasion, such a sentence will be criticized as 半文不白 (mixed style of classical and vernacular Chinese). However, compared with a single 乃, the compound verb 乃是 sounds a limited little more modern, so 意识乃是大脑的一种自然属性 is OK.
In your giving sentence, it means is (be)
but as a chinese ,this verb is hardly use in communicating or writing, this is a verb which uses in ancient chinese
My suggestion is that you'd better have a Chinese dictionary when you learn it, for example <新华字典>. All the usual meaning will be presented with example. The 乃 in your example has the same meaning in the following example:吾乃常山赵子龙也from （三国演义）.
