You can use regular detergent, but it isn't recommended, especially for a front loader. Front loaders don't drain all the way like top loaders do. They may use 1/3 of the water that top loaders use, but they still don't drain it all. HE detergent is more concentrated and it uses less of the flowery stuff that other detergents use. Because of this, it creates less suds that would potentially block the draining. Over time, you'll spend more money using regular detergent anyway because of the concentration. I'd say to suck up the cost a bit because it will save you money and mildew in the long run.
On a side note, try leaving your washer door open when it's not in use so air can get inside. That'll severely lessen your chances of getting mildew, especially if you leave it open right after a steamy load.
Wow, that last sentence sounds gross, but I can't phrase it any other way :)
No. Regular detergent has too much sudsing action. That will result in pump damage. HE detergent is also easier on your clothes because it has less salt.Washing machine hose
You are best using High Efficiency detergent.
However, if you have some detergent to use up you can usually get away with it on laundry that isn't too dirty and just use much less.Why do clothes turn inside-out in the washing machine?