Complex sentence: Sheldon is a senior theoretical physicist at Caltech and did share an apartment with his colleague and best friend, Leonard Hofstadter (Johnny Galecki), until Sheldon moves across the hall with his fiancée Amy, in the former apartment of Leonard's wife Penny.
Split into simpler sentences: Sheldon is a senior theoretical physicist at Caltech and shared an apartment with his colleague and best friend, Leonard Hofstadter (Johnny Galecki). However, Sheldon now lives across the hall with his fiancée Amy, in the former apartment of Leonard's wife Penny.