There were 6 birds.
2 birds flew away.
Now there are 4 birds left. (The 4 birds are left)

此時left必須做後置定語

如果要說那6隻鳥在樹的左邊不能說be left，只能說be to the left (of ...)
如：
The 6 birds were to the left of the tree.

Now the 4 birds are to the right of the tree.
同樣也不能說be right


be right的意思是「什麼什麼是正確的」
比如
You are right.

有時候根據語境可以把介詞to換成on。
on the left