Complex sentence: Swedish naval units required to support the attack started to arrive in mid May however since the naval commanders had been ordered to patrol between Reval and Helsingfors they refused from following Lewenhaupt's orders.
Split into simpler sentences: Swedish naval units required to support the attack started to arrive in mid May. However, since the naval commanders had been ordered to patrol between Reval and Helsingfors they refused to follow Lewenhaupt's orders.