Transmitters and Receivers
The transmit chain
Follow the signal: oscillator, buffer/amplifier stages, driver, final amplifier, low-pass filter, antenna socket. The oscillator sets frequency; the amplifier stages raise power; the filter removes harmonics. A question that asks where a spurious signal is reduced is usually pointing at the output filter, not the microphone amplifier.
Keying and modes
Know the difference between CW (on/off keying), SSB (amplitude variation of a suppressed carrier), and FM (frequency deviation). A frequent trap is calling FM a constant-amplitude mode with no bandwidth: FM occupies bandwidth set by deviation and modulation frequency. Another is confusing modulation with amplification.
Interference and spurious outputs
Harmonics are multiples of the wanted frequency; they are reduced by filtering and by good transmitter design. Overdeviation in FM, overmodulation in SSB, and excessive drive all cause splatter and key clicks. The correct answer is nearly always "reduce drive/modulation and check the filter", not "increase power".
Receivers
A receiver needs sensitivity, selectivity, and stability. Sensitivity is the ability to hear weak signals; selectivity is the ability to reject adjacent signals; stability is freedom from drift. The trap is treating them as synonyms. A receiver with poor selectivity hears strong nearby stations even when sensitivity is good.
Worked example
A transmitter on 14.2 MHz produces a strong signal at 28.4 MHz. That is the second harmonic. The fix is a low-pass filter at the output and checking the antenna match. Rule of thumb: if the unwanted signal is an exact multiple of the wanted one, think harmonic and filter.