Viewport question

I can go in any orthographic view(top, left...) by rotate + hold shift.
But why I can not back to perspective view by rotating? Any settings may be?

That's definitely a setting causing the issue.  When I snap to 2d ortho with shift-rotate, all I have to do to get back to 3d perp/ortho is shift-rotate again.  I honestly can't remember if this is the default, or if I customised these keys (I did most of my keybinds ages ago).  The order you press the keys is important... so I have to press alt-lmb (to rotate) and THEN tap shift.  I can't press shift, followed by alt-lmb, because that creates a box selection operation instead.

Thank you, now it works!!