Prepositional phrases are relatively easy to learn. Simply put, a prepositional phrase is a preposition plus whatever answers the question "What?" after it.

The following list includes most of the words that can function as prepositions:
 

Having found a word on the list in a passage, form a question with that word followed by "what," i.e., "since what?"

If whatever in the sentence answers that question forms a sentence of its own, the construction is not a prepositional phrase: 

Otherwise, it is:

      Your brain already knows what prepositional phrases are. It has to, since you use these phrases correctly day in and day out. What you are trying to do here is to make that unconscious knowledge conscious so you can understand how prepositional phrases fit within sentence patterns.

      You will, of course, make some mistakes, but you are expected to. Prepositional phrases can become complicated when they involve other constructions. As you learn about these constructions, the prepositional phrases that involve them will become clearer. The majority of prepositional phrases, however, are relatively simple.

      Another reason for beginning with prepositional phrases is that they account for so many syntactic connections. Here, for example, is how E.B. White describes the road to "The World of Tomorrow":
 

Here is the same passage with the prepositional phrases marked. 
 

Admittedly, this sentence is exceptional: sixty of its sixty-five words are in prepositional phrases. My research indicates that the norm for professional writing is 38%, not 92%. What this means, however, is that simply by identifying all the prepositional phrases in a passage, you will have accounted for more than one third of the syntactic connections in it. YOU WILL BE ONE-THIRD OF THE WAY TOWARD OUR GOAL!

     Little words cause the biggest problems. For students who cannot identify verbs, the difference between "to" as a preposition and "to" as the sign of an infinitive can be a real headache. Remember that you are expected to make mistakes. This is one area where they are expected. As you work your way through the levels, you will become more familiar with verbs and will find this distinction easier to make. Unfortunately, this "exception," i.e., that "to" plus a verb is an infinitive, itself has an exception: if the verb functions as a gerund (almost always ending in "-ing"), you ARE dealing with a prepositional phrase:

Prepositional Phrases:
They went {to the store}.
She returned {to the house}.
He likes every sport {from table tennis} {to hunting}.
Not Prepositional Phrases (Infinitives):
They wanted to go.
They went to help the victims.
To err is human.

"Than"

      Some people consider "than" to be a subordinate conjunction, not a preposition. A complication occurs because the ending of the subordinate clause is usually not stated (It is ellipsed.), or so say the grammar books. Consider:

Traditional textbooks tend to claim that this means:

I'd like to know how the writers of the textbooks know that. Since I always ran slow, I certainly would not have meant that. Some grammarians would counter by saying that the sentence actually means:

Another example, however, suggests that that argument doesn't work either. "Sally is prettier than Sarah" does not mean "Sally is prettier than Sarah is [exists]." (The adjective preceding the "than," in other words, has to remain part of the equation.) Nor does it have to mean that either woman is pretty.

      Most people's minds probably often process "than" as a preposition. Thus we would hear "Sally is prettier than me." and "He runs faster than me." While we are working on prepositions, I let students analyze these as prepositional phrases. I don't encourage it; instead, I simply ignore what they do with "than." If students ask, I do say that it is o.k. to consider it a prepositional phrase, but I note that there is a complication with the word "than." In my own research, I would mark "than me" as a prepositional phrase. 
     For the sake of curiosity, I checked the Merriam-Webster's On-line Collegiate Dictionary. Its entry for "than" as a preposition includes an interesting summary of this little debate. There is a usage question here, but my position is that if Shakespeare and T.S. Eliot could use "than" as a preposition, I have no right to tell my students that they can not.

      When we get to clauses, we face the problem. A cute young woman once wrote:

This causes a problem because the ellipsis doesn't work right. Since "me" is in the objective case, the sentence means

I could picture the young men in the course imagining how they would try to train her. But since we are now working with clauses, students are able to see the ellipsed clause structure. And, since our analysis ALWAYS depends on meaning, students can decide for themselves whether they are dealing with a prepositional "than" or an ellipsed subordinate clause.