A prepositional phrase is called that, and not a noun-and-preposition phrase or the like, for a good reason: The preposition determines the meaning. Here are five examples of pairs of prepositional phrases that are distinct in meaning because of the preposition used.
1. At Ease/with Ease
In addition to being a military command to prompt a parade formation to adopt a degree of relaxation, “at ease” refers to a state of relaxation: “He stood at ease while waiting his turn.”
“With ease” also modifies an action, but it is used to refer to a sense of effortlessness in such statements as “She completed the gymnastics routine with ease.”
2. At the End/in the End
“At the end” is not used on its own as an idiom, but it appears in such expressions as “at the end of the day” (to mean “ultimately” or “as it turned out”) and “at the end of (one’s) rope” — or “tether” — to mean “pushed to (one’s) limits.”
“In the end,” by itself, has the same senses as “at the end of the day.”
3. In Favor of/in Favor With
To be in favor of is to approve of something. To be in favor with a person or a group is to have their approval or support.
4. By Name/in Name
“By name” is used in such constructions as “I don’t know him by name” — meaning that the person in question is known by the speaker or writer on some other basis, as by sight. (The speaker or writer recognizes the person by his appearance, for example, but doesn’t know his name.) Byname is a synonym for nickname.
“In name” appears in the phrase “in name only,” meaning “not actually” (“Smith is the leader in name only; Jones actually runs everything”). “In (someone’s) name” means “as (someone’s) property” (“The house is in my name”) or “on (someone’s) behalf” (“Stop! In the name of the law!”)
5. On Time/in Time
“On time” means “punctual.” “In time” means “within a determined time.”