Refer to figure.
QNH: The atmospheric pressure adjusted to mean sea-level.

If you are on the ground at an airport and turn in the local QNH at your altimeter’s subscale, it will display the airports altitude above mean sea level.

QFE: The pressure set on the altimeter so that it indicates the height above the reference elevation being used (height above current ground level).

If you set the QFE on your altimeter’s subscale on the ground, the altimeter would read 0 feet.

QNE: The indicated altitude at the landing runway threshold when 1013.25 is set in the altimeter’s subscale.

In other words, it is the pressure altitude at the landing runway threshold.

QFF: Mean sea level pressure, derived by taking into account the actual temperature conditions.

So, the QFE reduced to mean sea level with regard to the actual outside air temperature.

If the airfield is at MSL => QFE, QNH and QFF will all be the same pressure. The temperature could be more or less than +15°C and the pressure could be more or less than 1013.2 hPa; it doesn't have to be ISA conditions. Let's now see why:

• QFE is the pressure value at the airfield;
• QNH is a QFE value which has been brought to MSL using ISA conditions;
  If you were to use actual conditions, QFF would be obtained instead.

Imagine that you are in an airfield at MSL, you measure QFE and obtain i.e. 1020 hPa
Now, reduce it to MSL with standard conditions to obtain QNH. You are already at MSL, the value is the same.
Reduce it to MSL with real conditions to obtain QFF => again, you are already at MSL, the value will not change.