We can sometimes replace a relative pronoun and finite verb with an infinitive. This is sometimes called a relative infinitive clause, or infinitival relative clause. This happens more often with defining relative clauses, but can also occur with non-defining clauses:
- The first person to speak at the conference was an expert on ...
(= the first person who spoke ...) - Jenny is definitely somebody to keep an eye on.
(= somebody who you should keep an eye on) - The chemist gave her some tablets, to be taken three times a day.
(= which should be taken / were to be taken)
When can we do this?
There doesn't appear to be a lot of information about this in standard EFL books, but there seem to be two main contexts where we can use an infinitive in a relative clause.
The first gets some space in advanced grammar books, but the second gets hardly a mention, at least not in the context of relative clauses.