There are four factors which decide whether a relative pronoun (or the word that) can be omitted or not:
- Is it a defining relative clause?
- Does the main verb in the relative clause have a separate Subject?
- Is the relative pronoun the first word in the relative phrase? (or is it preceded by another word, for example a preposition)
- Is the word who, which or that?
If the answer to the questions above is yes then the relative pronoun can be omitted. There are lots of duff websites around which might tell you that you can only omit these words if they represent the Object of the relative clause. This is hogwash. You can omit the pronoun as long as it is not the Subject of the matrix verb in the relative clause. The pronoun can, for example, be omitted if it is the Object of a preposition or Subject of another clause embedded within the relative clause.
Here are some examples to illustrate each point. An asterisk, *, denotes an ungrammatical example.
We cannot usually drop a pronoun from a non-defining relative clause:
- The agent I met up with wants you to phone him. (defining)
- *Your father, I met up with yesterday, wants you to call him. (non-defining)
We cannot drop the relative pronoun if the main verb in the relative clause does not have a separate Subject:
- I don't like the elephant you bit. (matrix verb in r-clause has a Subject, you)
- I don't like the elephant you said bit you. (matrix verb in r-clause has a Subject, you)
- *I don't like the elephant bit you. (verb in r-clause has no Subject)
Notice that in the second example the missing pronoun represents the Subject of the verb bit.
If the pronoun is embedded within another phrase, for example a preposition phrase, then it cannot be omitted:
- That's the circus I work in.
- That's the circus in which I work
- *That's the circus in I work.
We can drop the pronouns who, which and the word that, but we cannot drop the pronoun whose:
- That's the table I bought.
- That's the girl I like.
- That's the girl whose table I like.
- *That's the girl table I like.
The Original Poster's example
"For someone used to the tiny creatures we get in England it was something of a shock."
The word someone here has been post-modified by an adjective phrase. Some people argue that this is the result of removing who is from a relative clause. If you have a defining relative clause which uses the verb BE, you can often drop the relative pronoun and the verb BE. Whether it is now a kind of relative clause, or just an adjective phrase, or participle phrase modifying the noun is up for debate. Here are some more examples:
- The man [who was] going into the chip shop was an undercover agent.
- The elephant [who was] interested in the buns was rather plump.
Here's an example where you can't:
- The people who were blond preferred detective fiction.
- *The people blond preferred detective fiction.
We often cannot do this if what's left of the clause is only one adjective.
The sentence has a real relative clause modifying the phrase tiny creatures:
- ... creatures [which] we get in England
This is a defining relative clause, and the verb get has its own Subject, the word we. There are no other words preceding which. We can therefore happily drop the relative pronoun.
We could rewrite the sentence like this to show where potential words have been omitted:
For someone who was used to the tiny creatures that we get in England it was something of a shock.
Many grammars use the terms restrictive/non-restrictive or integrated/supplementary to describe what I've called defining and non-defining relative clauses.