These now are the walls （主句：主系表结构）we must tear down.（定语从句：修饰the walls）

关于now：

now 是个时间副词，修饰are。正常位置应在are 之后，本句提前是为了强调。比较：

1. These now are the walls we must tear down.

2. These are now the walls we must tear down.

还可以放在句首，具有提示作用：

3. Now these are the walls we must tear down.

若把now 放在句末，则修饰tear down：

4. These are the walls we must tear down now.