主语加系动词加动词ing形式的句子结构如下：

动词ing形式作为主语

动词ing形式可以表示经常性或习惯性的动作，通常用作句子的主语。例如：

Reading books is a good habit.

Playing basketball can exercise your body.

Swimming can improve your lungs.

Doing is not so easy as saying.

Flying in the sky is great fun.

Singing is better than saying.

动词ing形式作为谓语动词的宾语或伴随状语

在某些情况下，动词ing形式可以作为谓语动词的宾语或伴随状语。例如：

Mary enjoys listening to classical music.

Peter has finished doing his homework.

Do you mind opening the window？

Alice sat at the （此处应填入动词ing形式，但具体形式未给出）

It is no use/no good等后接动词ing形式

在一些固定表达中，如“It is no use/no good/useless/worthwhile/dangerous/a waste of time/fun等”，后面通常接动词ing形式作为真正的主语。例如：

It is no use shouting.

It is no good crying over spilt milk.

这些例句展示了动词ing形式在不同语境下的用法，既可以作为主语，也可以作为谓语动词的宾语或伴随状语。希望这些信息对你有所帮助。