The sentence uses present tense verbs, so "could not" must be present tense as well— that is, "cannot." Remember to check that verb tenses coordinate in a sentence!

The underlined words make up an incomplete verb phrase, as the verb "become" is in the wrong form. They appropriate version of "become" is the perfect tense, which indicates what has already happened. "Have become popular" is the correct answer choice.

The sentence describes what the soldier had already done to his own body. The use of "himself" is correct and necessary to show who the action was done to, but the action occurred in the past, meaning the appropriate verb form is "backed." The correct answer choice is "had backed himself."

Although the verb is next to the singular noun "their father," the subject is actually compound, "The boys and their father," and therefore plural. The correct verb needs to be a plural form. "Eat" is the best choice among the answer choices.

The phrase "until they were changed" contains a past tense verb and indicates that the use of the present tense verb "prevent" is incorrect. "Prevent" needs to be changed to the past-tense "prevented" to make the sentence grammatically correct. "The arrangements prevented the groups from developing" is the only answer choice that uses the correct verb tense.

The use of "have," a past-tense helping (auxiliary) verb, makes the verb in the sentence past-tense and perfect. However, "sing," a present-tense verb, completes the signaled past perfect form incorrectly and introduces confusion about the sentence's tense. While we could change the verbs in the sentence in many ways to correct the error, only "always sing" is underlined. This means that we can't alter "have," so we need to change "sing" to "sung" to create a correct past perfect verb ("have . . . sung"). The answer choices "always sing" and "sing always" don't correct the sentence's error. "Always sang" may look potentially correct, but "to sing" is an irregular verb in which "sang" is the past tense and "sung" is used in the perfect tense (Example: "She sang the song yesterday, after she had sung the duet."). "Always sang" can't be the correct answer, then, because it incorrectly constructs the past perfect tense. While "always sung" and "sung always" may each look like a potentially correct answer, "sung always" introduces an awkward word order that "always sung" avoids, so "always sung" is the correct answer, making the corrected sentence, "We have always sung anthems that celebrate our nation."

The sentence describes what Transcendentalists did "In the nineteenth century." This indicates the underlined portion of the sentence needs to make use of the past tense, because that part of the sentence is describing the actions that took place "In the nineteenth century." Only "seeking" needs to be changed, as the form "to find" is an infinitive, and does not change regardless of the tense of the verb that precedes it. "Sought to find" is the correct answer choice.

We know that the proper tense to describe an event that happened "over the course of past year" is the present perfect, which narrows down potentially correct answer choices to either "have increased the company's profitability" or "has increased the company's profitability." 

Since the subject of the increasing event is "wave," not "advances," we should go with the answer choice that uses the singular verb—"has increased the company's profitability."

The underlined portion describes what happened "Previously," which indicates the verb has to be in a tense showing what happened before. The correct answer must be in the past tense, and only "refused to run" has the correct tense among the answers.

The original sentence uses the pluperfect verb form ("had gone to the gas station") erroneously because the event of going to the gas station happened after Miguel left the store.