Keyboard input source changes on its own

The Keyboards with buttons as per Apple layout, viz. fn, control,option,command, are not available. So my first question is whether a Standard Keyboard for Windows computer will be compatible? I ran "Apple Diagnostics". It did not detect any problems with the Hardware. Then, why the built-in Keyboard is throwing up problem? Thirdly, I tried to start the laptop in "Safe Mode". However, even after follwing the described procedure (holding the Shift key immediately after pressing the power button) I wasn't able to boot in "Safe Mode" as no "Safe Boot" was visible in the upper right corner of the window. Meanwhile the original problem persists.