Counting prepositions depends heavily on what you include. A commonly cited range of totals is 100–150, and Cambridge English Grammar Today puts the figure at over 100. Within that larger total, the core set of frequent single-word prepositions is roughly 50–70, including items like in, by, and between, while multi-word forms such as because of account for much of the rest. One published list counts 142 English prepositions, but that source does not state its inclusion criteria, so it should be read as one count among several rather than a fixed upper limit. These different totals are not really contradictory; they simply draw the boundaries of the class differently. What is stable is that the class is closed, so new members appear almost never.
Recognizing the five functional categories: place, time, direction, manner, and agent
Ask one question about each word: does it say where, when, which way, how, or by whom? If it answers where something sits, it is place; if it pins a moment or period, time; if it shows movement toward a destination, direction; if it names a means or style, manner; if it names a doer, agent.
Place answers where: The keys are on the table. On marks physical contact with a surface, so the touching surface is the clue that points to place.
Time answers when: We met on Monday. On attaches to days and dates, so a nameable day or date in the sentence is the clue that points to time.
Direction answers which way: She walked to the station. To points at a destination rather than a resting position, so a moving subject and a goal are the clues that point to direction.
Manner answers how: They spoke with confidence. With links an action to its means or style, so if the word names how something was done rather than where or when, manner is the category.
Agent answers by whom: The novel was written by Austen. By names the doer behind a passive action, not a place or tool, which is the clue that separates agent from manner.