Complex sentence: In September 1939 Johnston joined the 2nd Battalion Grenadier Guards, and was sent for officer training to Sandhurst after which he was posted to the Grenadiers Training Battalion, based at Windsor, in the spring of 1940 where he served as a Technical Adjutant.
Split into simpler sentences: In September 1939 Johnston joined the 2nd Battalion Grenadier Guards, and was sent for officer training to Sandhurst. Following this, he was posted to the Grenadiers Training Battalion, based at Windsor, in the spring of 1940 where he served as a Technical Adjutant.