As a noun:
* "The infix "-ing" is used to form the present participle in English."
* "In some languages, infixes are used to change the meaning of a word."
* "The infix "-er" is used to create comparative adjectives."
As an adjective:
* "The infix position of the morpheme "un" in "unhappy" makes it a prefix."
* "The infix vowel "i" in "bitter" makes it a different word than "biter."
As a verb:
* "The linguist infixed the morpheme "un" into the word "happy" to create "unhappy."
* "Some languages infix words to change their meaning."
Remember that the specific way you use "infix" will depend on the context of your sentence.