关系子句常会出现介词，而关系代词是该介词的受词。在口头上，介词通常放在关系子句的句尾，关系代词可以省略。在正式英语上，介词会放在关系代词前，而关系代词不得省略。在以下例句中，写在括号内的关系代词表示可以省略。
举例
| 口头英语 | 正式英语 |
|---|---|
| Is that the man (who) she arrived with? | Is that the man with whom she arrived? |
| Does he know the girl (that) John is talking to? | Does he know the girl to whom John is talking? |
| The person (who) he is negotiating with is the Chairman of a large company. | The person with whom he is negotiating is the Chairman of a large company. |
| It is a club (which) many important people belong to. | It is a club to which many important people belong. |
| He liked the people (that) he lived with. | He liked the people with whom he lived. |
| The tree (that) they had their picnic under was the largest and oldest in the park. | The tree under which they had their picnic was the largest and oldest in the park. |
| It was the river (that) the children preferred to swim in. | It was the river in which the children preferred to swim. |
| The jungle (that) the tribe lived in was full of strange and unusual animals. | The jungle in which the tribe lived was full of strange and unusual animals. |