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'Who' is the subjective case relative pronoun used for people (the children). In this sentence, it serves as the subject of the verb 'are' in the relative clause. 'Which' would be used for things, and 'whom' for objects.

'Who' is the correct relative pronoun to refer to a person (the boy) when the pronoun is the subject of the relative clause. Here, 'who' is the subject of the verb 'was'. 'Whom' is the objective case and would be incorrect here.

'That' is a relative pronoun used for things (the knife) in a defining clause. It identifies the specific object that the speaker requested. 'Who' and 'whom' are only used for people.

'That' is used here to refer to 'the knife'. It introduces the relative clause 'you have bought' which defines the specific object being discussed. 'Who' and 'whom' are incorrect as they refer to people.

 They who seek wisdom...We use 'that' as a pointer or in conjunction with indefinite pronouns (all, each etc) or with superlatives (most, best), none of which is applicable in this case.

The relative pronoun 'whom' is used here because it follows the preposition 'by' and serves as the object of the preposition. 'Who' is the subjective case, while 'whom' is the objective case required in this grammatical structure.

In this sentence, the relative pronoun acts as the subject of the verb 'seek'. Therefore, the subjective case 'who' is the correct choice to refer to 'They'.

The relative pronoun is the object of the verb 'saw' (we saw the juggler). In formal grammar, 'whom' is the objective case used to refer to people when they are the object of a verb or preposition.

Case of sentence formation. If the precedant is in the past perfect tense, the antecedant must be in present perfect tense. Hence, option 2.

Case of coordinative conjunction. 'Lest' is always followed by 'should'.

Serving our parents is a 'responsibility' that comes from within. Modal 'should' is the best choice to depict a responsibility.

This is a tricky sentence. In sentences like this, we prefer usage of 'dare', but this sentence is followed by a question mark (?) and thus, 'can' is the preferred usage.

This sentence demonstrates the 'ability of past'. Modal 'could' (which is past of 'can') is the best option for the same purpose.

This is an imperative sentence. Somebody is given an advice for not to do something. Moreover, the sentence is in the present tense. So, modal 'should' is the best option.

'Hope' means probability. And for probability we use either 'may' or 'might'. Verb 'thought' in the sentence shows that the sentence is in the past and 'might' is the past of 'may'.