Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 172; Flight Crew Total 20040; Flight Crew Type 7540
Training / Qualification; Time Pressure; Situational Awareness; Communication Breakdown
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Environment - Non Weather Related; Human Factors
I was the pilot monitoring and my First Officer was the pilot flying from ZZZ to ZZZ1.  The First Officer briefed and the flight computer was programmed for the ILS XXR.  We were flying to ZZZZZ waypoint on ZZZZZ4 RNAV STAR.  When the Approach Controller assigned us the ILS XXC approach; the First Officer incorrectly executed the secondary flight plan.  He asked for my confirmation; but he had already executed the secondary plan before I had a chance to tell him not to do so.  And; he misinterpreted the FMA on his PFD; which led him to make an incorrect turn using the heading mode selector.  This caused a course deviation.  He was confused.  So; I reprogrammed the flight computer to rejoin the arrival route.  Soon after; ATC gave us a heading to intercept the localizer for XXC.  The flight was completed safely and normally.  After the flight; I briefed him on the importance of confirming with the crew before executing the flight computer program; not to be in a hurry; to take time to correctly interpret FMA; and to know how to correct your own mistake when you make one.  During this COVID pandemic period; he has moved from this fleet to XXX fleet then back again to this fleet.  That might help explain his getting confused on this flight computer system.  He took the post flight briefing professionally; and I believe it will help him in the future.  However; I must say that we cannot keep using the pandemic as an excuse for errors and mistakes made on the job.  Professionals should; regardless of the root cause; be able to recognize his or her own deficiencies; and continue to put effort into doing a better job every time.
I was probably off course temporarily while on the ZZZZZ arrival while we were attempting to reload the arrival and changed runway.
Air Carrier flight crew reported a track deviation due to the First Officer's confusion with the FMC. Reportedly; the First Officer has changed aircraft during the pandemic several times which may have contributed to the event.