I think that I can swim.
I can swim就是that引导的宾语从句，而且是表示陈述句的，不是表疑问或感叹或祈使句.
1、He says that he wants to speak to the headmaster.他说他想和校长说话
2、 I think that we need to buy some meat for the party.我认为我们需要为聚会买些肉.
3、 I'm sorry(that)he isn't here right now.对不起，此刻他不在.
