椅子下面怎么说用英语

The area under a chair is typically referred to as the "underside" or "bottom" of the chair. You could also describe it as the "base" or "legs" of the chair. For example, you might say:
"I need to vacuum the underside of the chair."
"There's something stuck to the bottom of the chair."
"The base of the chair is wobbling."
"The legs of the chair are uneven."
I hope this helps! Let me know if you have any other questions.