A preposition is a word that makes a connection or shows a relationship between a noun (or something functioning as a noun) and the element that comes before it in a particular sentence. When we say, “the milk in the refrigerator,” the word “in” is a preposition coming directly before the noun “refrigerator,” and it tells us the relationship between the milk and the refrigerator. That relationship is “in,” not “on,” “near,” “with,” “behind,” etc. Prepositions are called “prepositions” because their position in a sentence is the position just before (“pre-“) a noun (or noun phrase). Prepositions play an extremely important role in the connecting of elements in a sentence.
Prepositions often connect nouns with preceding nouns (“the chairs in the living room”; “the meeting at noon”; “the saddle on the horse”), but they also connect nouns with other (preceding) elements in the sentence. When a preposition connects a noun with a preceding noun, the preposition often shows a clear relationship between the noun following the preposition and the noun coming before the preposition. Sometimes, however, the preposition serves equally or principally as a simple connector between nouns and other (preceding) elements of a sentence rather than as a clear signal of a certain type of relationship between the elements. (Of course, to the native speaker, the choice of preposition may seem somewhat clear or just a little subtle, but to the non-native speaker, the choice may not be clear at all.)
She was happy with her gift. (ADJECTIVE + PREPOSITION + NOUN)
They were happy in their decision. (ADJECTIVE + PREPOSITION + NOUN)
He apologized for his offense. (VERB + PREPOSITION + NOUN)
This pattern is similar to that one. (ADJECTIVE + PREPOSITION + NOUN)
In sentences like the above, the element that precedes the preposition controls the choice of the preposition. For example, the use of the word “similar” requires that the preposition “to” be used to link it with the next element. The meaning of the preposition as an individual word can be a factor in its being the correct choice, but sometimes the choice is entirely limited by the grammar. In other words, sometimes we have to use a certain preposition after a certain word because that is what the grammar rule for that particular word dictates. We do not say, for example, “I’m proud with you,” we say “I’m proud of you.” It is the rule for using the word “proud” that decides that the preposition “of” must be used before the upcoming noun.
Some words allow more than one preposition to follow them. For example, we can say “She was happy with her gift,” and we can also say “She was happy about her gift.” However, the choice of preposition makes a difference to the meaning of the sentence. (“She was happy with her gift” sounds as if she has received her gift and she likes it. “She was happy about her gift” sounds as if the idea of the gift is a happy thing for her whether she has received the gift yet or not or whether the gift is even something that is real.)