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I learn English grammar using a textbook. Now I'm learning how to talk about necessity. In the textbook there is a part about obligation; how to use 'must' and 'have to' to express obligation. Then there is a part about necessity. The grammar rule from this part is:

'We can use 'need to', 'must' and 'have to' to express a necessity that results from things other than just commands, rules or laws, or to show that something is very important'.

Commands, rules and laws are discussed in the previous part about obligation. The authors tries to distinguish these two parts I think.

Now my question. Can I really use 'need to', 'must' and 'have to' interchangeably to talk about necessity?

Here are my some thoughts:

  1. I think because 'must' and 'have to' are also used to express obligation they are just stronger or formal than 'need to'.

  2. I know we can use 'need to' when something is necessary for our health or body. So I think when talking about health it's more preferable to use 'need to', not 'must' or 'have to'.

  3. I also know it's possible to use 'have to' to talk about necessity when I cannot choose not to do something. For example: 'I have to leave now. The taxi is here'. So in this way it's better to use 'have to' not 'must' or 'need to'.

These observations give me a sense that this rule is quite strict and simple, in my opinion.

Hope to find the answer here.

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    Short answer: Yes, you can. In fact, "I need (must or have to) to leave now". all mean the same thing.
    – Lambie
    Feb 2 at 15:31

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Beware of turning explanations into rules. The sentence from the textbook is very broad, sort of an introduction to the topic. It's not meant to suggest that all three phrases are appropriate for all situations of talking about necessity.

Your thoughts are good. Yes, "must" and "have to" are generally stronger than "need," since they express obligation, while "need" expresses... well, need. If we want to get philosophical, "need" is internal—it's about what I feel a lack of—while "must"/"have to" are external—they're about restrictions or demands that others place on me. But beware, these are still broad generalizations. It's easy to come up with examples that blend or cross these lines.

For instance: I'm late for a meal and very hungry. I could say "I need to eat": It's true, my body needs food. But I could also say "I must eat." No person is forcing me to, but I suppose what I really mean is "I must eat or else my hunger will become unbearable and that option isn't acceptable to me so in some sense I'm being compelled." Or, going the other way, I could say "I have to pay my taxes": laws and agencies force me to. But I could also say "I need to pay my taxes." Both sentences have the same broad meaning—"this is something for me to do"—but the emphasized meanings are a bit different. "Need to" expresses it as a task to accomplish, just one of the things I "need to" do like buy dinner, and subtly implies that the task aligns with my inclination, while "have to" expresses it as a burden and subtly implies that I might not be as willing.

You could also use "need to" as a euphemism to socially soften a "must/have to" situation. I own a store and someone tries to leave without paying: "Excuse me sir, you need to pay for that." Often people soften direct orders by phrasing them as personal requests: "I need you to stop hitting your brother now."

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  • Do I correctly understand? I can use 'have to' or 'must' in your example saying "I have to/must pay my taxes" to express it as task to do, as a necessity. Without any context, just this sentence, it's not so clear whether I mean it as a task I need to accomplish or something external, a law I have to obey. But within context it's clear what I mean, even though it seems less informal and casual than 'need to'. Feb 5 at 14:55

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