Complex sentence: When Margaret Thatcher was elected Britain's first female Prime Minister on the Fourth of May 1979, her party to place an advertisement in The London Evening News which said: May The Fourth Be With You, Maggie.
Split into simpler sentences: When Margaret Thatcher was elected Britain's first female Prime Minister, it happened on the Fourth of May 1979. This led her party to place an advertisement in The London Evening News, which said: May The Fourth Be With You, Maggie.