OK, some assumptions to be made here since not all countries adhere exactly to ICAO. Since you have posted from the UK, we'll assume that you are flying an aeroplane in UK airspace by day and hold a UK (not JAA) PPL.
++++++++++ Question 1 ++++++++++
Visual Meteorological Conditions (VMC)
In Class C, D, E, F or G airspace, above FL100 - a minimum of 8km flight visibility, 1000ft vertical/1500m horizontal separation from cloud
In Class C, D, E, F or G airspace, below FL100 - a minimum of 5km flight visibility, 1000ft vertical/1500m horizontal separation from cloud
In Class C, D or E airspace, below 3000ft AMSL - a minimum of 5km flight visibility, clear of cloud and in sight of the surface
In Class F or G airspace, below 3000ft AMSL and less than 140kt IAS - a minimum of 1500m flight visibility, clear of cloud and in sight of the surface
There are no VMC minima for Class A airspace since VFR flight is not permitted
There is no Class B airspace in the UK
Instrument Meteorological Conditions (IMC)
Any meteorological conditions less than VMC minima
Visual Flight Rules (VFR)
Within Class C, D or E airspace (controlled airspace) - Rules 25 & 27 of the Rules of the Air Regulations 1991:
1. Maintain VMC (see above)
2. File a flight plan
3. Obtain ATC clearance
4. Maintain radio listening watch
5. Obey ATC instructions
Within Class F or G airspace (outside controlled airspace) - Rule 26 of the Rules of the Air Regulations 1991:
1. Maintain VMC (see above)
2. errrr - that's it
Instrument Flight Rules (IFR)
Within Class C, D or E airspace (controlled airspace) - Rules 29, 31 & 32 of the Rules of the Air Regulations 1991:
1. Fly at or above Minimum Safe Altitude
2. File a flight plan
3. Obtain ATC clearance
4. Fly in accordance with the clearance obtained and any notified SID, STAR or holding procedure
5. Inform ATC of any deviation from the clearance obtained
6. Inform ATC on leaving controlled airspace or landing within it
7. Make position reports as notified or required by ATC
Within Class F or G airspace (outside controlled airspace) - Rules 29 & 30 of the Rules of the Air Regulations 1991:
1. Fly at or above Minimum Safe Altitude
2. Obey the Quadrantal or Semicircular Rule as appropriate
++++++++++ Question 2 ++++++++++
The holder of a UK PPL who does not also hold an Instrument Rating or IMC Rating cannot fly in conditions "which require compliance with the Instrument Flight Rules". It is permitted to fly under IFR in other conditions but it is difficult to see any point in doing so by day. (ANO Schedule 8, Section 1, para. (2)(e) & (2)(f))
++++++++++ Question 3 ++++++++++
Two things govern VFR flight above cloud; VMC minima and licence privileges. VMC is achieved 1000ft vertically clear above cloud and licence privileges are maintained if the pilot is "in sight of the surface" (ANO Schedule 8, Section 1, para (2)(c)(iii))
++++++++++ Question 4 ++++++++++
Whilst the AIP does state that, "To ensure the greatest possible degree of safety and uniformity" pilots should use QFE for the approach, most companies use QNH and, in practice, ATC at large airfields will normally assume that to be the standard. Smaller airfields where most pilots are PPL holders continue to use QFE as the norm for approaches and circuits.
Any spare tea in that pot?