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

此时left必须做后置定语

如果要说那6只鸟在树的左边不能说be left，只能说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.
同样也不能说be right


be right的意思是“什么什么是正确的”
比如
You are right.

有时候根据语境可以把介词to换成on。
on the left