For example, "I have a job" and "I need a job" are opposite statements, but "I have to get a job" and "I need to get a job" are the same. I understand that they are no longer modifying the same words, but it doesn't seem to make sense why the meaning switches.
Learn English – Why do the sentences “I have to get a job” and “I need to get a job” mean the same thing if ‘have’ and ‘need’ are antonyms
antonyms
Best Answer
Have to is a periphrastic modal idiom. Need is a semi-modal verb. Must is a modal auxiliary verb.
First, have to is an idiom.
Have to is not a usage of the verb have, in any of its senses, though it used to be.
You can tell this because have to is pronounced differently from have two, for instance.
This idiom is a periphrastic modal -- an idiomatic construction that means the same as a modal auxiliary verb, in this case, the verb must. So
although have to and must are quite different with negation
Second, need is an odd verb in a number of ways; it's called a "semi-modal" verb because it used to be a modal auxiliary verb, but, along with dare, it can now only be used as a modal auxiliary verb
But ... need can only be used this way in a negative context. I.e, it's a Negative Polarity Item (NPI).
(the second example in each case is the modal usage; the first is not, and
thus requires Do-Support, present tense marking, and an infinitive with to.)
So need to is just the ordinary usage of an ordinary verb need, which does not mean (though it isn't quite an opposite of) have in its sense of 'possess'. It does, however, mean pretty much the same thing as have to or must; all of them have similar deontic senses meaning 'be obliged to' for one reason or another.
Have to and must also have epistemic senses, dealing with logic and probability; however, need seems not to have an epistemic sense. This may have been lost along with need's syntactic modal superpowers as it slipped from full modal to semimodal over the centuries.