In this sentence, there is an illogical comparison between Maria’s birthday party (a thing) and Susie (a person). Assuming that the comparison should be between two birthday parties, the corrected sentence reads: "Maria had a gigantic bouncy castle at her birthday party, making it significantly more popular than Susie’s party."

In the above sentence, two subjects are being compared: the knight and the samurai. When a sentence compares only two subjects, it is appropriate to use the comparative form ("more") rather than the superlative form ("most") of adjectives.

This sentence uses a superlative (“most”) when it is only comparing between two things. The comparative “more” would be better. The corrected sentence reads, "I used to find information about public transportation on the printed schedules, but I have discovered that the online ones are the most helpful."

"Less than" should be changed to "fewer than." When deciding whether to use "less" or "fewer" in relation to a give noun, if you can count that noun or it has a plural ("people," "glasses of water," etc.) "fewer than" is grammatically correct, and if you can't count it or it has no plural ("time," "money," "water," measurements) "less than" is grammatically correct. As an example, you would say "less water," but "fewer buckets of water."

The phrase "the more expensive of the three" contains this sentence's error. Three types of sweaters are being compared: acrylic sweaters, wool sweaters, and cashmere sweaters. The comparative adjective "more" is used; this is incorrect because comparative adjectives are only used for comparing two things, as in the sentence, "I like vanilla ice cream more than chocolate-covered strawberries." When comparing three or more things, a superlative adjective should be used. In this case, that means that "more" should be changed to "most" to correct the sentence's error.

Because the baby is being compared to all of the babies in the world, the superlative "cutest baby" is needed. If the baby were being compared to only one other child, the comparative "cuter baby" would be appropriate. This makes the correct selection "cuter baby."

Because only two objects (the train and the airplane) are being compared in this sentence, the comparative "faster" must be used. If there were three or more items being compared, the superlative "fastest" would be appropriate. This makes the correct selection "is the fastest."

In this sentence, there are nine boxes, so the the superlative “heaviest” would be better than the comparative “heavier.” The corrected sentence reads: She stumped me with a riddle about ten boxes of various weights; the goal was to find the heaviest one by only using the scale twice.

As a general idiomatic rule, the verb "protest" always goes with the preposition "against," not "over." Therefore, the phrase should be "protest against," not "protest over."

The preposition here should be spelled "through." "Threw" is the past tense singular form of the verb "to throw" and is not appropriate in this sentence.