We must keep our gloves on. We don’t want to get frost-bitten.=
We must keep our gloves on so as not to get frost-bitten.
why not:
We must keep our gloves on not to get frost-bitten.?
The farmer put a scarecrow up in the field. He wanted to frighten the birds.
The farmer put a scarecrow up in the field to frighten the birds.
why not:
The farmer put a scarecrow up in the field so as /in order to frighten the birds.?
3.well,what is the difference between:
a)‘not to do sth’ and ‘so as not to do sth’?
b)‘to do sth’ and ‘so as to do sth/in order to do sth’
c)does ‘in order not to do sth’ exist?
“in order to” and “so as to” can be shortened to “to”. For example, the following are all possible and mean more or less the same. (In this particular example, “so as to” feels less natural to me, but let’s ignore that as it is not central to your question.):
“The farmer put a scarecrow up in the field in order to frighten the birds.”
“The farmer put a scarecrow up in the field so as to frighten the birds.”
“The farmer put a scarecrow up in the field to frighten the birds.”
However, “in order not to” and “so as not to” cannot be shortened to “not to”.
“We must keep our gloves on in order not to get frost-bitten.” – OK
“We must keep our gloves on so as not to get frost-bitten.” – OK
“We must keep our gloves on not to get frost-bitten.” – WRONG
