Complex sentence: The maiden flight of the ARJ21 was planned to take place in 2005 with formal handing over of the aircraft for use 18 months afterwards; however, the design work was delayed and the final trial production stage did not begin until June 2006.
Split into simpler sentences: The maiden flight of the ARJ21 was initially planned to take place in 2005 with commercial service beginning 18 months afterwards. However, the design work was delayed and the final trial production stage did not begin until June 2006.