How do you say she was doing her homework IN FRENCH?

There are a few ways to say "she was doing her homework" in French, depending on the context and level of formality:

Formal:

* Elle faisait ses devoirs. (This is the most common and neutral way to say it.)

Informal:

* Elle faisait ses devoirs. (This is also acceptable in informal settings.)

* Elle bossait ses devoirs. ("Bosser" is a more informal word for "work".)

Adding specificity:

* Elle était en train de faire ses devoirs. (This emphasizes that she was in the process of doing her homework.)

* Elle s'est mise à faire ses devoirs. (This emphasizes that she started doing her homework.)

Ultimately, the best way to say it depends on the context of your conversation.