Ah, good old Facebook. How did we ever communicate without it? We love it, but trying to take advantage of some of its capabilities can be confusing.
You don’t offer a lot of detail in your question. Are you trying to tag a friend on facebook status, or in a picture or a post? Are you new to Facebook or have you been able to tag in the past and this is a new development?
Let’s say you are new to Facebook. Are you sure you are following the steps to tag correctly? If you’re trying to tag someone in a post just start typing their name within your post. A drop down list should appear of friends whose names contain those letters. Just click on the friend you want to tag. You can choose to tag one or more by typing more names and repeating the process. This method should work for either typing a status update or commenting on someone else’s post.
If that doesn’t work for you when you’re doing a status update, look at the lower left hand corner of the box. See the four small icons there? The one, second from the left, allows you to tag someone on your friends list. Click it and a box will pop up asking who you’re with. Start typing the name in there and again the dropdown list will appear. Just click on the friend of friends you want to tag.
If you want to tag someone when sharing a post that you want them to see, hit share, choose the share icon, and select a friend’s timeline and start typing.