短语讲解：

1. In intransitive verb patterns, adverbs or prepositional phrases are often referred to as "optional."

refer to 参考; 指的是; 涉及
例句：We often refer to these books in our work.
我们工作中经常参考这些书。

2. In other words, a sentence is still grammatical without them.

in other words 参考; 指的是; 涉及
例句：I mention this point now in passing and shall refer to it again.
这一点现在顺便提一下，以后还会讲到。