Bevestigend: have/has + voltooid deelwoord (ww+ed / 3e rijtje)
I have walked to school since... - He has had pizza recently.
Vragend: have/has vooraan
I have walked to school since... - Have I walked to school since...
Ontkennend: haven't / hasn't
I haven't walked to school since... - He hasn't had pizza recently.