I am new at using QGIS. How can I simply split features in QGIS? The manual tells to simply draw a line through the polygon you want to split. But it doesn't work. The line disappears when finishing it. When I select the polygon and then split it, it sometimes works and sometimes doesn't (an info appears after which it does not split the feature). Does anybody have an idea why the tool does not work properly?
If you want to split your features per hand: Select your vector layer and click editing (red). Then select the scissors(green) and click two-times from one side of the polygon to the other. You will see a small dotted line. After spliting simply unable the editing and choose "save changes". Look into the attribute table to see the changes.
To update Curlew above, as of QGIS 2.18, you would use the built-in Advanced Digitizing Toolbar, which can be accessed via View > Toolbars > Advanced Digitizing Toolbar. The toolbar will look like this and the "Split Features" button is the fifth from the right:
With that loaded, and the respective later selected and in Edit mode, select the "Split Features" tool and use it to draw through the place(s) where you want to create segments. You can then use the "Node Tool" to select vertices and move them wherever the new segments should be positioned. A quick summary looks like this:
In the end, each segment will then become an independent feature in your vector layer. Be sure to save your edits!
For more information, see the QGIS user manual section on advanced digitizing.
In addition to Curlew's answer it's important to start and end your splitting line outside of the target layer. I carefully aimed for the nodes until I found that useful tip here:
The most important thing is to have a clear difference between right and left part of the polygon to be split across a splitting line. Otherwise it would not work. So be sure there is a "clear" border between right and left or upper and lower part even if this mean that you make unwanted cuts. These you can merge afterwards to the main parts.