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I dont understand these two, it seems like abit same, and different x_X, please help/teach me. Sighz

(a) The relative pronoun must agree in number (singular or plural) with the word immediately preceding it.

Example: Incorrect: She is one of those girls who is always boasting.
Correct: She is one of those girls who are always boasting.

(b) The verb and subject of a clause or sentence must always agree in number (singular or plural) and person (1st, 2nd, 3rd).

Example: Incorrect: A wide range (of goods) were displayed.
Correct: A wide range (of goods) was displayed.
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She is one of those girls who is always boasting.

She is a girl who is always boasting. If you say one of that means there are more than one girl and you give us the example of the one in this group so , in the example of She is one of those girls who are always boasting who is the relative pronoun of those girls not she.

A wide range (of goods) was displayed. A wide range of is considered as a unit rather than something can be counted one by one, for this reason we use is with a wide range of.
It is better for you to learn by heart such things.They may not have a rule every time.
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It would make more sense to present these in the reverse order.

1. Subject and verb agree in number (singular or plural).

The girl is always boasting. [girl (the subject) is singular; is (the verb) is singular.]
The girls are always boasting. [girls
(the subject) is plural; are (the verb) is plural.]

2. A relative pronoun takes the number (singular or plural) of its antecedent.
(Antecedent is what your book means by 'immediately preceding' word.)

(3. If the relative pronoun is also a subject of a clause, it will affect agreement with its verb by the principle stated in 1. above.)

She is the one who is always boasting. [one (the antecedent) is singular; who (the relative pronoun; the subject of the clause) is therefore also singular; is (the verb) is singular.]
She is one of those girls who are always boasting. [girls (the antecedent) is plural; who (the relative pronoun; the subject of the clause) is therefore also plural; are (the verb) is plural.]

CJ