Identify the incorrect phrase in the sentence: "The teachers let the students to know that they would have an hour to complete the test."

The phrase "to know" is incorrect in this context because it disrupts the grammatical structure required after the verb "let." The verb "let" is typically followed by a direct object and then a base form of a verb, known as the infinitive without "to." In this case, the correct construction should simply be "let the students know," where "know" is used in its base form after "let." This aligns with standard English syntax, ensuring the sentence maintains clarity and proper structure.

The other portions of the sentence function correctly: "let" serves as the main verb, "would have" appropriately indicates the future tense, and "the test" is a correctly formed noun phrase. Thus, the error lies with the unnecessary use of "to" before "know," leading to a grammatically incorrect phrase.