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: 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' 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' 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.
We use the past perfect to show that an action happened before another action in the past. It can also show that an action happened before a specific time in the past.
When we arrived at the station, the train had already left.
Had Carol studied Russian before she moved to Russia?
We form the past perfect with: had/hadn’t + past participle
+ She had seen the film before.
- She had not seen the film before.
Future Perfect Continuous has two different forms: "will have been doing " and "be going to have been doing." Unlike Future Simple forms, Future Perfect Continuous forms are usually interchangeable.
The Future Perfect is formed with ‘will have + past participle’ but ‘be going to have + past participle’ has the same function and these two forms are interchangeable.
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.