To familiarize students with the commonly used prefixes of un- and pre-, have them sort words containing these prefixes. Give students a collection of cards labeled with words that begin with the prefixes un- and pre-, such as "unfair" and "unzip" or "preheat" and "preview." Invite them to read each word and sort the cards by their prefix. As a class, discuss the meaning of each word, pointing out how the prefix changed each root word.
Gather the students together to do a class activity using root words and prefixes. Prepare several cards labeled with root words and place them in a large hat. Prepare another set of several cards labeled with the prefixes un- and pre- and place them in another hat. Invite one student to pull a card from the prefix hat and one from the root word hat and place both cards on the board for all students to see. Determine as a class if the word you created is a real word or not, and if it is ask students to write the word on a list. The teacher could also have students choose words from the list and use them in a sentence.
After students have become familiar with the prefixes un- and pre-, they can begin to find them in words they read. As you read aloud, invite the students to listen for words containing those prefixes and create a list of them together. Discuss the meaning of each word in the context of the story you are reading. During independent reading time, give students paper to create a list of words they found containing the prefixes. After students are done reading, gather them together to see who found the most words with prefixes.
Using the same cards from the hats activity, pass one card out to each student so that some have a root word and some have a prefix. Make sure that you use root words that can make a real word with one of the prefixes, un- or pre-. Ask students to mingle around the classroom looking for a person with a card that can be combined with theirs to make a real word. For example, a person with the root word "believable" would be looking for a person with the card "un" to make the word "unbelievable."