Zero identifying clause

A zero relative clause is an identifying relative clause from which the relative pronoun has been left out (ellipsis), for example: This is the book I bought cf. This is the book that I bought.

In other words, the pronoun (that/who/which) can be left out when it is the object of the clause. This structure is particularly common when the pronoun is followed by is/are/was/were:
 * An extrovert is a person [who is] concerned more with external reality than inner feelings.

Examples

 * Everything [that] I like is either illegal, immoral or fattening.

Specific difficulties
Students with Spanish as L1, while probably understanding the structure when they come across it, will be averse to using it themselves as Spanish requires use of that "missing" pronoun.