I know it's hard to understand mental illness. The brain, our most important organ, and yet the one known the least about. If we try to break it down, and instead think of a different organ in our body that is malfunctioning, say for instance, the digestive tract, you can be vomiting one minute, and OK the next. You can be vomiting, yet still thinking. Vomiting, yet still able to walk around, speak and say what foods are making you sick, or telling someone to grab a trash can for you. You may be vomiting for hours, and then, you're not. But maybe next month, or 6 months from now, you're vomiting again. It can be from different causes. It can be from different causes in the same episode of vomiting. You've gotten food poison, while also have gall bladder issues for instance.
Much of the same happens with a brain medical issue. You may be depressed, yet still be able to go to work, still laugh at a joke, still be an effective parent. Or you maybe unable to start the task of taking a shower, changing clothes, leaving the house... there's so many variables.