MODAL AUXILIARY

Helping verbs or auxiliary verbs such as will, shall, may, might, can, could, must, ought to, should, would, used to, need are used in conjunction with main verbs to express shades of time and mood. The combination of helping verbs with main verbs creates what are called verb phrases or verb strings. 

Function

The meaning are usually connected with ideas of DOUBT, CERTAINTY, POSSIBILITY and PROBABILITY, OBLIGATION and PERMISSION (or lack of these). You will see that they are not used to talk about things that definitely exist, or events that definitely happened.

These meanings are sometimes divided into two groups:

DEGREES OF CERTAINTY: certainty; probability; possibility; impossibility

OBLIGATION/FREEDOM TO ACT: permission,lack of permission; ability; obligation.

Pattern

S + Modal Aux + Verb

S + Modal Aux + Not + Verb

Modal Aux + S + Verb

Let's look at each modal verb separately, and the functions they help to express: WILL

Making personal predictions

I don't think the Queen will ever abdicate. 
I doubt if I'll stay here much longer.

Talking about the present with certainty (making deductions)

I'm sure you will understand that there is nothing the Department can do
There's a letter for you. It'll be from the bank: they said they'd be writing.

Talking about the future with certainty

I won't be in the office until 11; I've got a meeting.
Don't bother ringing: they'll have left for their 10 o'clock lecture.

Talking about the past with certainty

I'm sure you will have noticed that attendance has fallen sharply.

Reassuring someone

Don't worry! You'll settle down quickly, I'm sure.
It'll be all right! You won't have to speak by yourself.

Making a decision

For the main course I'll have grilled tuna.
I'm very tired. I think I'll stay at home tonight.

Making a semi-formal request

Will you open the window, please? It's very hot in here.
Sign this, will you?

Offering to do something

You stay there! I'll fetch the drinks.

Insistence; habitual behaviour

I'm not surprised you don't know what to do! You will keep talking in class.
Damn! My car won't start. I'll have to call the garage.

Making a promise or a threat

You can count on me! I'll be there at 8 o'clock sharp.
If you don't finish your dinner off, you'll go straight to bed!

http://library.bcu.ac.uk/learner/Grammar%20Guides/3.07%20Modals.htm

https://blog.udemy.com/modal-verbs/

https://www.tesol-direct.com/guide-to-english-grammar/modal-auxiliary-verbs

if clauses or conditional sentences

What is a conditional?

Conditional is a possible results from certain situations.

 

Types of Conditional Sentences

·         Type 1             :                       True in the present/future

·         Type 2             :                       Contrary-to-fact in the present/future

·         Type 3             :                       Contrary-to-fact in the past

Verbs in time clauses and conditionals follow the same patterns as in other clauses except:

• In clauses with time words like when, after, until we often use the present tense forms to talk about the future:

I’ll come home when I finish work.
You must wait here until your father comes.
They are coming after they have had dinner.

•  in conditional clauses with if or unless we often use the present tense forms to talk about the future:

We won’t be able to go out if it is raining.
If Barcelona win tomorrow they will be champions.
I will come tomorrow unless I have to look after the children.

• We do not normally use will in clauses with if or with time words:

I’ll come home when I will finish work.
We won’t be able to go out if it will rain. rains.
It will be nice to see Peter when he will get home gets home.
You must wait here until your father will come comes.

• but we can use will if it means a promise or offer:

I will be very happy if you will come to my party.
We should finish the job early if George will help us.

"if" clauses and hypotheses

Some clauses with if are like hypotheses so we use past tense forms to talk about the present and future.

We use the past tense forms to talk about the present in clauses with if :

• for something that has not happened or is not happening:

He could get a new job if he really tried	=	He cannot get a job because he has not tried.
If Jack was playing they would probably win	=	Jack is not playing so they will probably not win.
If I had his address I could write to him	=	I do not have his address so I cannot write to him.

 We use the past tense forms to talk about the future in clauses with if:

• for something that we believe or know will not happen:

We would go by train if it wasn’t so expensive	=	We won’t go by train because it is too expensive.
I would look after the children for you at the weekend if I was at home	=	I can’t look after the children because I will not be at home.

•  to make suggestions about what might happen:

If he came tomorrow we could borrow his car.
If we invited John, Mary would bring Angela.

When we are talking about something which did not happen in the past we use the past perfect in the if clause and a modal verb in the main clause:

If you had seen him you could have spoken to him	=	You did not see him so you could not speak to him
You could have stayed with us if you had come to London	=	You couldn’t stay with us because you didn’t come to London.
If we hadn’t spent all our money we could take a holiday.	=	We have spent all our money so we can’t take a holiday
If I had got the job we would be living in Paris	=	I did not get the job so we are not living in Paris.

 If the main clause is about the past we use a modal with have: 

If you had seen him you could have spoken to him.	=	You did not see him so you could not speak to him.
You could have stayed with us if you had come to London.	=	You couldn’t stay with us because you didn’t come to London.
If you had invited me I might have come.	=	You didn’t invite me so I didn’t come.

If the main clause is about the present we use a present tense form or a modal without have:

If I had got the job we would be living in Paris now.	=	I did not get the job so we are not living in Paris now.
If you had done your homework you would know the answer.	=	You did not do your homework so you do not know the answer.

source :

https://learnenglish.britishcouncil.org/en/english-grammar/verbs/verbs-time-clauses-and-if-clauses