Question

'Today she weighs 21 stones' make the sentence negative

Answer 1

The negative sentence will be:
Today she does not weigh more than 21 stones.

Keep in mind that the meaning of the sentence should NOT be changed, whatever you do.

Answer 2

We do negatives by placing not after the initial segment of the action word:

Today she does not weigh 21 stones.

They are not buckling down.

They won't buckle down.

They had not buckled down.

They have not been buckling down.

They probably won't have been buckling down In communicated we regularly lessen don't: They aren't buckling down.

They won't buckle down They hadn't been buckling down and so forth.