is, are: The correct use of these words depends in a measure on the intention of the writer or speaker. Therefore, the choice of a singular or plural verb in cases where either form would be proper is often influenced by the writer's way of looking at the subject. "The purpose and conception of the scheme is to do good." Now the mistake with this sentence is that either "purpose and conception" represent a single idea (in which case they may, in combination, take a singular verb), or they do not (in which case they require a plural verb), and that in the former case, where the nouns express a similarity of sentiment, one of the words is superfluously used. "Jones and Smith is solvent": yes, as a firm, though as individuals they are solvent.
it: Used sometimes in such manner as to violate the principles of grammatical and rhetorical construction, as when referring to any one of several words or clauses preceding, or perhaps to some idea merely implied or hinted at in what has gone before, as in the following: "A statute inflicting death may, and ought to be, repealed, if it be in any degree expedient, without its being highly so." In this sentence "if it be" should be replaced by "if such repeal be," and "its" should be omitted.
In general, personal and relative pronouns with ambiguous reference to preceding words or clauses