Ronald and Lois married for two decades consider themselves a happy couple.

Ronald and Lois 主语

married for two decades 非限制性定语

consider 谓语

themselves 宾语

a happy couple. 宾语补足语

这个句子本身就是完整的句子。married for two decades为形容词短语（或理解为过去分词短语）作非限制性定语。通常应该在其前后打逗号。如下句：

Ronald and Lois, married for two decades, consider themselves a happy couple.

译文：结婚二十年的罗纳德和罗伊斯自认为是幸福的一对。/ 罗纳德和罗伊斯结婚二十年了，他们认为自己是一对幸福的夫妻。

marry可以是及物动词，也可以是不及物动词。其过去分词表示结婚的动作发生后所处的已婚状态，词典已将married 收录为形容词，意为已婚的。过去分词或形容词表示已婚状态，是可以用持续时间状语修饰的。married for+时间名词，可以作表语、定语或状语。以下为COCA和BNC语料库 married +for时间状语作表语、状语、定语的例句：

COCA

Married for almost 50 years, the Bornsteins have four children and 11 grandchildren. 

I was married for twelve years to a man who didn't love me. 作状语

Brown, 47, told Us. " We were married for 14 years.“ 作表语

London's Telegraph says a couple married for sixty-seven years died just hours apart. Family members say that Hugh and Joan Nees couldn't bear to be separated. 作限制性定语

BNC

I was married for a couple of years in the seventies. 作状语

Elaine -- married for 23 years -- said:' It's a blasted nuisance.‘ 作非限制性定语

Married for 31 years, Miss Clark spends much of her time at her flat in London's Knightsbridge, while Mr Wollf, who is also her manager, lives at their home in Paris. 作状语

In 1832, an English couple, the Pattisons, married for just one month, were drowned in it. 作非限制性定语