This probably refers to the absence or presence of the infinitive marker "to." In the sentence "I must go" the infinitive ( "go") lacks the marker, while in the sentence "I want to go" the infinitive has it.
The word "how" functions as a subordinating conjunction in the sentence "I know how to write." It introduces a subordinate clause that provides more information about the verb "know."
The infinitive phrase plays the role of an adverb in this sentence. It tells why you met at the park. In the sentence "You met at the park to run", "to run" is the infinitive phrase.
The infinitive in this sentence is "to audition."
The infinitive phrase is "to join the circus" (an adverbial phrase).
Yes. For example, "He could not decide what to eat."
An infinitive is preceded by the word "to". For example, in the sentence "I like to read," "to read" is the infinitive form of the verb "read."
The underlined adverb clause modifies an infinitive in the sentence: "She arrived early to win the race." In this sentence, "to win the race" is the infinitive phrase, and the adverb clause "early" modifies the purpose or reason for her arrival.
infinitive
The infinitive phrase in the sentence is "to help you". It functions as the direct object of the verb "would like".
The infinitive in this sentence is "to keep." It is the base form of the verb "keep."
Write into as one word. It is one preposition and should be treated as such. If perhaps in some strange sentence, the word to was part of an infinitive, then the in and the to would be separate. Maybe someone could figure out such a sentence but it would be difficult.