由 can be a noun,a prep.and a verb.
1,as a noun,由means reason or cause
When it be used as a noun,in morden Chinese,we usually combine 由 with other Chinese characters to become a word gourp(phrase)
such as 原由（yuán yóu），理由（lǐ yóu）
in fact,these word groups mean reason or cause.
2, as a prep,由 means from,due to，because of,thanks to ect.
When it means from ,it can be used as separately
石头由山上落下来（Stones came down from the mountain.)
When it means because of,thanks to ect.it must be combined with other Chinese characters in modern chinese.
such as 由于(means because of,thanks to,this word can be used as a derogatory sense word or acommendatory sense word)
The accident was due to careless driving（这场车祸是由于粗心驾驶造成的。)
3,as a verb,由means leave or entrust something to someone.it can be used as a single Chinese character
这件事由你负责（You are responsible for this matter)
又 can be used as a adv.or a conj.
when it be uesd as a adv. it means once again, also, again
eg:乔来了, 又喝醉了.(Here comes Joe, drunk again.)
and as a conj.
又means both ... and
eg:他既是一个战士又是一个诗人。(He is both a soldier and a poet．)
You can find when it used as a conj.there is a phrase:既（jì) ……又
in english this phrase means both ... and
In fact,learning Chinese word groups is more important than recite Chinese characters.because different Chinese characters in different word group its meaning will change.