Preschoolers readily understand the time concepts of before and after if you can provide a set schedule for them. Hold snack time and nap time at the same times every day. For instance, when your preschooler asks when you are going to go to the park, you can respond and say "after snack time." A set routine makes preschoolers feel secure, as they know the order of how they will do things during the day -- such as have a snack, eat dinner or play with their friends.
Point out the time on a digital clock and explain when certain events during the day will occur, and what the clock will look like at that time. For instance, if you are going to the park at 4:30, tell your preschooler that when the first number is 4, the second is 3 and the last is zero, that is when you will go to the park. Do this every day, so that she begins to comprehend that the changing numbers on a clock refer to the time of day that you do things. Once she learns how to read a digital clock, you can teach her how a clock with a minute hand and an hour hand works with a simple clock face. Make a simple clock face out of a paper plate, two arrow-shaped strips, a short one and a long one in different colors, and affix them to the paper plate with a metal-pronged brad fastener.
When a holiday or someone's birthday is approaching on the calendar, create a paper chain with a number of links equal to the number of days until the holiday or birthday. Cut as many strips of construction paper as there are days. Help your preschooler assemble the strips into a chain by placing the tape onto each strip as they bend them into a circle. Place the chain where she can see it readily every day. Help her remove a link every morning until the day of the event. This helps her learn about the passing of days and how to count them.
You can find several free online interactive games for preschoolers that teach them about time at ABCLearningTime.com, KidsOLR.com and Lil-Fingers.com. Play the games with your child to reinforce what a clock looks like. As you play, talk about what the time is on the clock as she figures out how to play the games.
Read books about time to your preschooler. You can find books that discuss the days of the week, the passing of the seasons, clock facts and so on. Preschoolers often love to have their parents read books to them, and doing this while reading about time provides the repetition needed for them to start learning the concepts.