A preposition links a noun phrase with the rest of the sentence. For example: I am sitting on the roof. The word “on” is a preposition in this sentence linking “roof” with the rest of the sentence. I am going to the house. The word “to” links “house” with the rest of the sentence.
Oftentimes, prepositions can be compounds. For example, I am going onto the roof. The word “onto” is a compound of two basic prepositions, “on” and “to”. A roofer might say this as he is about to climb a ladder and install roof tiles for a homeowner. I am going onto the roof. I’ll finish before sunset.
A lot of prepositions are formed from roots and affixes. For example, the affix “-side” is often added to create prepositions. Inside. Outside. Beside. Alongside. The prefixes “a-” and “be-” are often used to create prepositions. About. Abroad. Along. Amid. Among. Beneath. Below. Before. Beyond. Behind.
English prepositions are generally a closed class, meaning that new words are rarely added. We stick to our guns. There are some prepositions that sound archaic or old-fashioned, and are usually not used in every day speech. For example, some prepositions formed from the affix “-st” are old fashioned. Amid versus amidst. Among versus amongst.