As you use Audacity Insert Silence will be an editing skill you will need. As with many effects and edits in Audacity, there is more than one way to insert silence with Audacity. I will cover three Audacity Insert Silence techniques.
The insert silence editing technique I use mostly is the Amplify effect. Audacity Amplify effect does not actually "Insert" silence. Instead it takes a piece of sound you do not want and make that silent. This is great for editing out bad sounds. Here is how you do it: Use the Selection Tool to select the sound you do not want. Next go to the EFFECT menu and select AMPLIFY. There are a couple of ways to adjust the volume of your sound sample in the Amplify effect window. The easiest way is to use the slider you see in the Amplify window. Slide it all the way to the left. This will greatly reduce the volume of the sound sample you selected. They may not completely erase the sound, so you may need to do it twice to achieve complete silence.
Let's look at another Audacity Insert Silence technique: Generate Silence. If you have a recording created in Audacity and you need to put in a moment of silence, but you do not want to delete any of the sound you have already in your Audacity project then you can use Generate Silence. Place your cursor where ever you would like the silence then go up to the Generate menu and select Silence. In the next window you can choose exactly how much silence you would like to insert. When you are done click OK. Piece of cake!
The final way to have Audacity insert silence for you is by using the Split Track function. If you have a sound sample that needs some silence in it, but you do not want to delete any of the sample this method will work for you. The result is much the same as Generate Silence, but you can fine tune it a little easier.
Place your cursor where you would like to have Audacity insert silence. Next go to the EDIT menu, select CLIP BOUNDRIES -> SPLIT. This will break you track into two parts. Now use your Time Shift Tool to move the two portions apart creating silence between them. (Video Guide to Audacity has a great tutorial on the Time Shift Tool - Get it here!)