lie—to lie down

Present—The dog lies under the table.

Present—The dog is lying under the table.

Past—He lay upon the bed.

Past—He has lain there for hours.

Past—He has been lying there for hours.

lay—to put a thing down

Present—The boy lays his books on the table.

Present—The boy is laying his books on the table.

Past—He laid his head upon the block.