It's the boy's dream (play) soccer.

很高兴与你共同分享英语知识,
1、It's the boy's dream 【to play】 soccer.
原因：
固定句型结构——
It +be+名词【the boy's dream 】+（for sb）to do sth
其中,it是形式主语,真正的主语是【（for sb）to do sth】不定短语/不定式复合结构.
例如：
It‘s a good idea to start right now
=To start right now is a good idea.
这个句型中,【名词】还可以换为【形容词】.
It's nice of you to say so.
2、
I still have trouble__the test.A.to understand B.understand C.understanding D.understood
选C.
固定句型搭配：
have trouble
difficulty (in可以省略) doing sth
a problem
problems
意思是：在做某事方面有麻烦/困难/问题.
高考链接：
With the boy leading the way,we had no difficulty finding the village.
祝你开心如意!