when to use getApplication(), getApplicationContext(), getBaseContext() and someClass.this in Android.

Here, someClass.this, getBaseContext and getContext() are activity contexes while getApplicationContext() and getApplication() are application contexes.
Toast and Intent, both requires reference to context. And getApplication, getApplicationContext,LoginActivity.this and getBaseContext, they all offer reference to the context.
Now the thing confuses is the declaration of different contexts and their specific-usage. To make things simple, you should count two types of context available in the Android framework.
1.    Application Context
2.    Activity Context
Application context is attached to the application's life-cycle and will always be same throughout the life of application. So if you are using Toast, you can use application context or even activity context (both) because a toast can be raised from anywhere with in your application and is not attached to a window.
Activity context is attached to the Activity's life-cycle and can be destroyed if the activity's onDestroy is raised. If you want to launch a new activity, you must need to use activity's context in its Intent so that the new launching activity is connected to the current activity (in terms of activity stack). However, you may use application's context too to launch a new activity but then you need to set flagIntent.FLAG_ACTIVITY_NEW_TASK in intent to treat it as a new task.
Now referring to your cases:
LoginActivity.this though its referring to your own class which extends Activity class but the base class (Activity) also extends Context class, so it can be used to offer activity context.
getApplication() though its referring to Application object but the Application class extends Context class, so it can be used to offer application context.
getApplicationContext() offers application context.
getBaseContext() offers activity context.

Tips: Whenever you need to manipulate Views then go for Activity-Context, elseApplication-Context would be enough.

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