Mostly we use verb+了 to express pp. form of verb.
If you use verb+ n., you could use it either before n. or after n., like 我吃饭了 or 我吃了饭
And if 了 appears in the end of sentence and no verb just before it, it will be used as modal particle. Like 这些事都是过去的了, we could also say 这些事都是过去的啦