连词"that"引导名词性从句作动词"said"的宾语，在这里是为了明确转述内容的范围
先将句子转为直接转述：
→But Juan Guiado said, "the aid would be let in, because it's about saving lives, and the official who block it are keeping medicine and food from people in need."
可以看到Juan的原话中已经带有一个连词"and"。
在做间接转述时，如果没有连词"that"的话，对方可能会认为转述的内容在连词"and"之前就结束。
→But Juan Guiado said, "the aid would be let in, because it's about saving lives," and the official who block it are keeping medicine and food from people in need.
在语法意义上，这里是分成2个等位的并列句进行转述的：
He said (that) the aid ... , and (he said) that the official ... .
2.书面语中应把介词放在关系代词前
先将定语从句复原成一个完整的句子看看：
→You can make progress in a career or an organization by a series of stages.
作为定语从句修饰"a series of stages"的时候，需要把它置换成关系代词"which"并置于被修饰的先行词之后：
→a series of stages which you can make progress in a career or an organization by (a series of stages)
但是严格来说，"by a series of stages"是一个整体，因此在书面写作中，把它作为关系代词前置的时候需要带上介词"by"：
→a series of stages by which(=by a series of stages) you can make progress in a career or an organization
追问谢谢，解释的很详细!
p2，你可以从后面开始读。大概可以翻译为通过做什么，你可以在那方面取得成就
2. 这个是定语从句，根据意思，是通过…可以…，所以用by which
least
20
people
lost
their
lives
in
the
air
crash
and,according
to
the
hospital,___was
the
pilot,who
died
soon
after
he
was
sent
to
the
hospital.
A.among
which
B.among
whom
c.among
them
D.among
what
解答：whom引导的非限制性定语从句，whom=遇难的20人，
among
whom
=在遇难的20人当中意思
因此答案为B