By Marina Pantcheva
Relative clauses are used to give more information about a particular part of the sentence, usually a noun.
(1) The picture that you bought yesterday is really nice.
When a noun is modified by a relative clause, the verb in the relative clause has to agree in number with the noun. When the modified noun is part of a complex phrase, one might be tempted to make the verb agree with the closest noun. However, this can sometimes change the meaning. Compare the sentences (2) and (3) below.
(2) One of the participants, who was sitting in the last row, asked to leave the room.
The participant who asked to leave the room was sitting in the last row.
(3) One of the participants, who were sitting in the last row, asked to leave the room.
All participants were sitting in the last row, and one of them asked to leave.
It is therefore important to pay attention to exactly which noun is modified by the relative clause.
(4) He is one of those professors who do not go to conferences.
Modified noun professors: he belongs to a particular type of professors that do not go to conferences.
(5) He is the only one of the professors who does not to the conferences.
Modified noun the only one: from all the professors, he is the only one who does not go to conferences.
If you wonder whether a certain noun is singular or plural, check this link.