介词与连词
Prepositions 介词
英语中的介词向句中引入名词成分 (nominals)，即名词 (noun)、代词 (pronoun)、动名词 (gerund)、名词短语 (noun phrase)、名词性从句 (noun clause)。
介词的形态
介词有时并非“一个”词，在句中引导名词成分的部分都可视为介词。介词的形态有：
Simple Prepositions (简单介词)
只有一个单词的介词如 in, at, on 等。
He sat on the chair.
The sun is above the clouds.
Double Prepositions (双介词)
由两个简单介词复合或组合而成的介词，如 into, upon, out of, from within 等。
It is up to us to find the answer.
The loud noise came from within the stadium.
注意，只有当两个介词引导同一名词成分时才是双介词，以下情形不是双介词。
My mum brought a roast chicken in on a tray.
我妈妈拿进一只放在托盘上的烤鸡。
这里 in 是 bring in (the room) 中的介词，而 on a tray 是介词短语作方式状语。再比如，
I sat down on the sofa next to Barbara.
我坐在沙发上挨着芭芭拉。
这里 down 是 sit down 中的小品词，on the sofa 是介词短语作地点状语。
Compound Prepositions (复合介词)
由名词、形容词或副词添加介词前缀而形成的介词，如 across, beside, along 等。
My car is parked behind the store.
There was a tunnel running underneath the city.
Participle Prepositions (分词介词)
分词形态的动词充当的介词，如 considering, pending, provided 等。
She is interested in anything concerning horses.
He works one job during the day and another at night.
Phrase Prepositions (短语介词)
含有介词的词组，通常首尾都是介词，如 because of, by means of, on behalf of 等。
On account of the snow, our plane was delyed.
For the sake of the children, do not use that language!
介词的功能
介词通常有以下功能：
引出时间
I started working here in 2009.
I haven’t played tennis for years.
引出方位
We’ll meet you at the entrance.
We walked for miles.
引出状态
Max’s on holiday this week.
The road is under construction.
引出原因
The bus was delayed due to heavy snow.
Because’ has become a preposition
引出原因
以介词收尾的句子
对于句子能否以介词结尾尚有争论。一般而言，因为介词必须引导名词成分，所以句子通常不以介词收尾。但有些情况下，刻意的介词前置会造成阅读困难，例如美国作家温斯顿·邱吉尔（Winston Churchill）曾抱怨到：
“That is the sort of thing up with which I will not put!”
就是这种东西我无法容忍！
该句为了不以介词结尾，将 put up with （小品词-介词短语动词）中的 up with 放在 which 之前，但影响了阅读体验。
牛津词典给出的建议是在以下4种情况下可以以介词结尾。
1. 被动结构（passive structures）
She enjoys being fussed over.
她喜欢被宠爱。
2. 关系从句（relative clauses）
They must be convinced of the commitment that they are taking on.
他们必须明确他们所承担的使命。
3. 不定式结构（infinitive structures）
Tom had no-one to play with.
汤姆没有玩伴。
4. 疑问句（questions beginning with who, where, what, etc.）
What music are you interested in?
你喜欢什么音乐？
Conjunctions 连词
介词与连词的区别
References:
https://examples.yourdictionary.com/preposition-examples.html
Ucdenver.edu/grammar/prep_vs_conj-intro.pdf
Grammarly.com/youve-been-lied-to-heres-why-you-absolutely-can-end-a-sentence-with-a-preposition
Yourdictionary.com/ending-a-sentence-with-a-preposition
Oxforddictionaries.com/grammar-myths-prepositions
https://www.hjenglish.com/new/p113589/
https://wenku.baidu.com/view/b47e7ffdff00bed5b8f31d89.html
To find more: