The issue with the sentence as written is the fact that its main verb does not match the number of the sentence's subject. The subject is "seeds." Think of a simple sentence to check the verb: "The seeds were . . ." If you were to say, "The seeds was," you would immediately be aware of the error! The intervening expression "for cultural restoration" can distract you if you are not careful. Now, no additional commas are necessary. The "for" is a preposition, not a conjunction.

The error in the original sentence is one of subject-verb agreement.  The subject is “the number,” with “of earthworms” serving as a prepositional modifier that tells us more about the number.  So the correct phrasing should be “the effect that the number (of earthworms) has on soil fertility…” Note that this occurs often on the SAT, with the authors of these questions adding descriptions (often in the form of prepositional modifiers) that include nouns, making your job of determining which subject goes with the verb a difficult one.

"Has on" is therefore correct.  Among the other answer choices, "it has on" is incorrect because of the pronoun “it” between the subject, number of earthworms, and verb, has.  And "can be to" uses improper diction: something can have an effect on something else, but to say that “its effect can be to” is an improper sentence structure.

This problem asks you to find the proper form of the verb “work,” and showcases a handful of important lessons.  For one, note that the verb “works” follows the relative pronoun “who” - this means that this verb applies specifically to the word that “who” is modifying, which is “curator.”  So while this sentence involves many nouns, the only one that matters here is “curator.” For that reason, the noun must be singular, eliminating choices "work" and "do work", which are plural.  

"Worked" is also incorrect, as the past tense implies that this curator in question has completed this work.  Note that everything else in the sentence is ongoing: the traditional view “involves,” the professor “spends.” Therefore to suggest that this one action is completed while the others are ongoing is illogical, making this incorrect.  The correct answer is "NO CHANGE", which uses a singular, ongoing verb “works.”

In this example, we need to seek out relevant cues from outside the underlined portion of the sentence to identify what is needed out of the underline. Here, the singular vs. plural nature of the subject, lion vs. lions, is at question. If we continue to the latter portion of the sentence, we find the pronoun “its.” Since “its” refers back to the subject and is singular in nature, the subject, “the lion,” must also be singular. This leaves us with only “the lion rules,” our correct answer, as “lions can be seen ruling,” “lions are able to be seen ruling,” and “lions rule” all utilize the plural subject, “lions.”

In this example, we are being tested on subject-verb agreement. In order to effectively eliminate wrong answers and arrive at the correct answer, we’ll need to determine the subject by addressing who or what “was/where/are/have been among the casualties. This case is particularly tricky, as it involves subject-verb inversion - where the subject comes after the verb. The subject of the sentence that is referred to as “among the casualties” is “a cluster.” We describe the cluster as “a cluster of farmhouses” to provide additional context, but the subject “a cluster” itself is singular, despite being accompanied by a plural noun in its modifying context. With this in mind, the only answer option that agrees with the singular subject “a cluster” is “was.” “Were,” “are,” and “have been” are all plural verbs that would require a plural subject.

In this example, we are being tested on subject-verb agreement. In order to effectively eliminate wrong answers and arrive at the correct answer, we’ll need to determine the subject by addressing who or what takes on each of our verbs: “have/has” at the start of our answer choices and “worry/worries” at the end. Using both logic and sentence structure, we can identify that what “have recently arrived on retailers’ shelves” are the products. So, the plural verb “have” is appropriate here, and we can eliminate all options that use the singular “has.”

From here, we’ll want to look to the subject taking on the verb “worry/worries.” If we read past or eliminate the modifying phrase “of the new products that ACME Corporation has developed over the past year and that [have] recently arrived on retailers’ shelves,” the core of the sentence reads “the quality worry.” Since quality is singular, the verb we need is “worries,” and we can eliminate any answer option that uses the plural “worry.” This leaves us with our correct answer: “have recently arrived on retailers’ shelves worries.”

In this example, we are being tested on subject-verb agreement. In order to effectively eliminate wrong answers and arrive at the correct answer, we’ll need to determine the subject by addressing who or what takes on the verb “have/has/have had/had.” If we break down the construction of the sentence, “of the members of the board” is a modifying phrase referring back to the singular noun, “each.” Since “each” is singular, we need singular agreement with our verb. Only “has” provides us with singular agreement, as all other options either use the plural “have” or seem to imply that each member had experience, but does not anymore, an illogical construction.

In this example, we are being tested on subject-verb agreement. In order to effectively eliminate wrong answers and arrive at the correct answer, we’ll need to determine the subject by addressing who or what takes on the verb “pose/poses/has posed/has the potential to pose.” In this case, “of a high-sugar diet” is a modifying phrase. The core of the sentence reads “effects poses.” since “effects” are plural, we need the plural verb “pose.” Each of our incorrect answers incorrectly utilizes a singular verb in some form, which does not agree with the plural term “effects.”

In this example, we are being tested on subject-verb agreement. In order to effectively eliminate wrong answers and arrive at the correct answer, we’ll need to determine the subject by addressing who or what takes on the verb “are increasing,” or its alternative options. Here, “of applicants” and “to top colleges and universities” are both modifying phrases. The subject of the sentence taking on the verb is “the number.” Since “the number” is singular, we need singular agreement, and only the option “is increasing” provides us with this singular agreement. We can also use logic for much of this question. Is the sentence trying to say that the universities are physically getting bigger? That the applicants are physically growing? Since both of these constructions are illogical, logic can point us to the correct and logical subject “the number.”

In this example, we are being tested on noun-pronoun agreement and parallelism. In order to effectively eliminate wrong answers and arrive at the correct answer, we’ll need to determine the subject by addressing who or what we’re referring to with the pronoun “its/their.” In this case, we have a powerful hint in the non-underlined portion - we already refer to Australia using the pronoun “its.” So, we can see that the collective noun, Australia, is singular. We also need to maintain parallel structure between what follows “not only” and what follows “but also.” Our correct answer, “but also for its” correctly refers to Australia using the singular possessive pronoun “its,” and introduces the phrase that follows the “but also” with the preposition “for” followed by the possessive pronoun and the object of possession - the same structure that follows the “not only” in the sentence.