Nancy is think of how to do it.改错

1、Nancy is thinking of how to do it.用现在进行时态“be doing”,在谓语动词后面加ing。译为：我正在考虑如何做这件事。
2、Nancy thinks of how to do it.用一般现在时，去掉is,同时在think后加s以保持主谓一致 。译为：我（经常）考虑如何做这件事。

think改成thinking，因为is是系动词，think是动词，系动词不能直接加动词，应将动词改成动名词

Nancy is thinking about how to do it.is后面用进行时，think about考虑

Nancy is thinking of how to do it.正在进行时用be doing