What's "that" and "which" in Chinese I know that for sentences such as "this is a port that has been closed for 20 years" I could use 的 or start another phrase.  Is it this way for "which" as well?
Yes, it is.
In Chinese, dependent sentences are not so common, and most phrases could be used as adj. with '的'.

e.g. "这是一个已经关闭了20年的港口"

Otherwise, you could also start another sentence to describe the meaning in 'which' clause, which could be separated by comma.

另外，你也可以用另外一句话来表述'which'从句中的意思，这一句跟前一句可以用逗号分开。