Learn English: What's the Difference between Should, Ought, and Must

hello everyone welcome back i'm teacher sarah  in today's video let's learn about modal verbs  

so today let's talk about three modal  verbs today we will look at the modal verbs  

should ought or ought to and must these words are  used in many different ways we can use these words  

to talk about duty and obligation obligation  is something that you have to do if you have  

an obligation it means no choice you have to do it  so we can use these words to talk about things we  

have to do we can use these words to give advice  and to say what we think is right for people to do  

or about what people have done our opinion about  what people have done if we think it is right  

so let's take a look at the many different  ways we can use these terms first let's look at  

should and ought these two verbs are very  similar and can often replace each other this  

means most of the time if i use the word should  i can also just replace it with the word ought  

or ought to although in english should is used  much more often than ought so if the word ought  

confuses you or you don't use it very often  that's okay you can use should in place of it  

so let's take a look at some examples of how we  can use should and ought to interchangeably or  

some examples that show that we can use these  words in just the same way first example  

you should see star wars it's a great series  

or you ought to see star wars it's a great series  so here i'm telling you i think it's a good idea  

for you to see star wars i think you should  do it i think you ought to do it same meaning  

here's another example you should join us  you ought to join us so here i'm suggesting  

or saying that um it's a good idea come on come  on you should join us you ought to join us so we  

can see in these two examples that we can use  should and ought to in very much the same way  

let's look at another way should and ought can be  used should and ought are also used to talk about  

logical probability uh what is logical probability  this means something that is most likely true  

if i think about it it's probably true  so let's take a look at an example  

i bought two pizzas that should be enough  i bought two pizzas that ought to be enough  

so let's say i'm having some friends over  and i decide to order some pizzas i think  

most definitely right really most probably that  two pizzas will be enough for all of my friends  

so i could say that should be  enough or that ought to be enough  

not quite the same as saying that is  enough uh maybe right there's a tiny  

tiny chance maybe we'll eat two  pizzas and we still want more but  

i look around and i think that should be enough  that ought to be enough let's see another example  

uh let's say i am waiting for my friend jack to  come over and finally i hear a knock on the door  

oh that should be jack knocking on the door or  that ought to be jack knocking on the door so i  

was expecting him if someone comes and knocks on  my door it is most likely most probably jack right  

i'm expecting him there's a small chance that  maybe maybe it's the the post man uh or maybe  

it's someone else but most probably it's jack ah  that ought to be jack so here we see we can use  

should and ought to talk about logical probability  or something that is most most most likely true  

okay next let's talk about the modal verb  must must has similar meanings to should  

and ought but is stronger and more definite  so let's take a look at some examples  

here's sentence number one the  doctor said i must give up smoking  

this is an order if your doctor says you  must give up smoking it means you you  

must you have no choice you are obligated to  do it you have to do it it is in order you must  

whereas if we use uh should or ought to here  you know you really ought to give up smoking  

this is just a suggestion or some advice  it is not someone telling you what to do  

it's just a suggestion so we can see  that must is a much stronger word

let's see another example of how must is stronger  than should an aunt if i say ada must be home now  

that means i'm sure she's home maybe aydah is  my daughter and i told her to be home by 7 pm  

if it's 7 30 i could say aydah must be home now it  means i'm sure she's home i told her to be there  

she must be there this is very definite right  i'm very positive very sure that she's there  

maybe if it's only 6 30 or 6 45 though i  could say aydah should be home now right i  

think she's home she should be she might be but  i'm not as sure so must is used when we are sure  

or something is definite or it's in order  so let's expand what we've talked about  

so far are the most common and most simple and  frequent use uses of these verbs so let's expand  

and see some other maybe more challenging  ways that we use these terms in english  

should can be used instead of must to  make orders and instructions more polite  

this is very common for businesses when they  are telling their customers to do something  

or even when teachers are telling their students  to do something they might use should instead  

of must or have to to be more polite for  example this form should be filled out in ink  

okay well you really have to fill it out in ink or  they'll make you do it again but by using the word  

should it sounds more polite here's a similar  example applications should be sent by may 31st  

well you must send them by may 31st or it won't  be accepted but to be more polite they should  

be sent by may 31st so we can use should to make  our instructions or orders a little more polite  

here's another expansion for the words should and  ought should and ought can be used for predictions  

but not must so a prediction is when we are  guessing what will happen right so for example  

i could say the weather should be fine tomorrow  or the weather ought to be fine tomorrow  

i'm predicting i'm guessing weather is a little  unpredictable maybe it will rain we don't know  

we think we're sure but not totally positive so  i could say the weather should be fine tomorrow  

let's go camping the weather ought to be fine  tomorrow let's go for a picnic but i cannot use  

must i would not say the weather must be fine  tomorrow because i can't tell the weather what  

to do right i can't tell the future what to do  so when we're talking about the future and making  

predictions we can just use should and ought all  right we have one more also about should and ought  

as you see there are so many uses for these words  uh this is the last one we'll talk about today  

should and ought can be used to talk about  unfulfilled obligations in the past so things that  

didn't happen but you should have done in the  past and again we cannot use must because it  

has already happened so for example let's say um  i'm talking to my friend she goes on a date with  

a guy and it doesn't go well she tells me about it  i might say hmm you should have been nicer to him  

so i'm saying this is something you should have  done in the past you should have been nicer to him  

i think this is how you should have changed  your behavior right you ought to have come  

to the meeting so you really kind of needed  to come to the meeting but you didn't maybe  

you missed a lot of information oh  you ought to have come to the meeting  

right so here we can also use should and ought to  talk about things that didn't happen in the past  

that you should have done but again we cannot  use must because must means it hasn't happened  

yet right if i'm telling you to do something you  must be nicer to him it means you ha it hasn't  

happened yet it didn't happen in the past and  i'm not predicting the future we can only use  

must in that very particular way so be careful  when you're using these kinds of sentences