by doing是什么时态

时态变化只体现在谓语动词上，和别的成分没有关系。

by是介词，后面不能是动词，如果是动词，要加ing。

例句

Can you do well by doing good?

你能通过做善事而事业有成吗？

Do it by doing things.

就要为了做事情而做事情。

Learning by doing may not be efficient, but it is effective.

边做边学可能效率不高，但却很有效。

My mother was told to lose weight by doing exercise every day.

我妈妈被告知要每天锻炼来减肥。

only by doing so can we maintain the vigor and vitality of our nation.

只有这样做，我们才能使的国家保持生机和活力。