1. I have not seen him since last January.
2. We rowed against the tide.
Since is a preposition, and shows the relation of time between January and have seen.
Against is a preposition, and shows the relation of opposition between tide and rowed.
To parse a preposition, tell:
1. What part of speech it is.
2. What relation it shows between its object and some other word in the sentence.
Parse the prepositions in the following sentences:
1. Let us go a fishing.
2. We dine about noon.
3. It is hard work to row against the tide.
4. They took up a collection for the poor.
5. The boys were allowed to play in the afternoon,
6. Take the load off the horse's back.
7. The water was pumped out of the well.
8. You should not act contrary to orders.
9. Those people came from beyond the seas.
10. A picture memory brings to me;
I look across the years and see Myself beside my mother's knee.