Complex sentence: Grey retained his position as Foreign Secretary when Asquith's Coalition Government (which included the Conservatives) was formed in May 1915, but when that ministry collapsed in December 1916 and David Lloyd George became Prime Minister, Grey went into opposition.
Split into simpler sentences: Grey retained his position as Foreign Secretary when Asquith's Coalition Government (which included the Conservatives) was formed in May 1915. When Asquith's ministry collapsed in December 1916 and David Lloyd George became Prime Minister, Grey went into opposition.