Vocabulary

'Might' is used mostly to express possibility. English speakers use 'might' to make suggestions or requests, although this is more common in British English and could be seen as extremely formal.
'Might' is also used in conditional sentences.

Should is used to give advice and to make recommendations. It is also used to express obligation and expectation.

Recommendation: When you go to London, you should go to the theatre.
Advice: You should try to focus more on your writing skills. Advice
Obligation: I should be going now. It's getting late.
Expectation: You should have understood the text by now.

Have to and must are being looked at together because of the inter-changeability when used for certain functions and the confusion caused when they cannot be interchanged for others.

Have to

'Have to' is used to express certainty, necessity, and obligation.
This has to be the right place. We are not lost. Certainty
The glue has to be left to dry for 24 hours. Necessity
I have to leave early. Obligation

'Could' is used to express: possibility, past ability, and to make suggestions and requests. 'Could' is also used in conditional sentences as the conditional form of 'can'.

The modal 'can' is a commonly used modal verb in English. It is used to express; ability, opportunity, a request, to grant permission, to show possibility or impossibility. It is this large amount of functions and the fact that ‘can’ is replaced by other modals when it is used to express future or past time that often lead to certain errors.

Here are some examples of 'can':

So + adjective

'So' when used with an adjective, shows extreme situations. This form is used mostly in speech:
The music is so loud! Why don’t they turn it down?
The hotel was so good. It was worth every cent.

Future Simple 'will + base form' and Going to

Expressing the future with 'will' or 'going to'

These two different forms for expressing future time can be used interchangeably sometimes. However they often express two very different meanings. It needs practice to understand the differences. Both 'will' and ‘going to’ refer to a specific time in the future.

Will – 'will + base form of verb'

You will see him later.
Will you see him later?
You will not see him later.

Used to + verb describes a past situation that is no longer true
I used to go scuba diving.
It is best to avoid using ‘used to’ in negative forms or questions although some native speakers do this in informal situations. It is better to use the Past Simple in those situations.

What's the difference between say, tell and ask?

Say

We say: hello and goodbye, please and thank you, happy birthday and congratulations.

Say hello to your sister for me.
We said goodbye at the airport.
Did you say thank you to Mrs Anderson?

We use say to ask about language:
How do you say 'car' in Portuguese? 'Carro'.