Modal verbs. Part 3 : Expressing obligation / duty / necessity.

Modal verbs. Part 3 Expressing obligation / duty/ necessity.



You can learn Part1 (modal verbs: general rules) and Part2 (abilities: can/ to be able to) here.






Must is used
- to show that someone is obligated to do something, it is someone's duty.
You must finish this work by Friday.

- to show that the speaker has decided that something is must be done.
I must finish this work by Friday.(I have decided)



Have to is used
-to show that someone else ( not a speaker) has decided that  the speaker must do something.
I have to finish this work by Friday. ( I don't want to, but I have to do this for any reasons)

-we also use have to instead of must for Past and Future tenses, because  must doesn't have Past and future forms. Learn more abot have to here

I had to finish this work by Friday.
I'll have to finish this work by Friday
Should /ought to is used 
to express strong advice or duty  to do something .

You should work hard to finish this work by Friday.(advice)
You ought to fihish this work by Friday . (it's your duty)

Need is used  to express the necessity to do something in questions.
Need I finish this work by Friday? 

This is how have to is used in everyday English :

 

'There are a few banana plants that flower and produce fruit every few years. Obviously it is difficult in the UK so you have to have a hot summer followed by a mild winter.

'The rest of the family have to suffer! They aren't big gardeners but they have to put up with what I've done to it, but actually they really like it and they love spending time in it.'

From Daily Mail

http://www.dailymail.co.uk/news/article-2688734/Businessman-Tim-Wilmot-spends-25-years-transforming-Bristol-garden-tropical-paradise-filled-bamboo-banana-trees.html#ixzz37BMjf3dC