Android Studio not showing modules in project structure

Update 19 March 2019

A new experience someone has just faced recently even though he/she did add a library module in app module, and include in Setting gradle as described below. One more thing worth trying is to make sure your app module and your library module have the same compileSdkVersion (which is in each its gradle)!

Please follow this link for more details.

Ref: Imported module in Android Studio can't find imported class

Original answer

Sometimes you use import module function, then the module does appear in Project mode but not in Android mode

So the thing works for me is to go to Setting gradle, add my module manually, and sync a gradle again:

First, create your Module. You can do that using the default Android Studio Module Wizard (File -> New -> New Module), select Android Library, give it a Name and a package name, and finish.After that you will see that nothing happens. Open the side Gradle Projects Section (should be somewhere on the right panel)

You should see something like this:

Expand your module gradle and run the Run Configurations task.After that refresh the content and refresh the project structure.

It should work now.

Do right mouse click on your project, then select 'Open Module Settings' - then you can add modules to your project..