Introduction actuality of the research paper

Ability in the present
‘Can‘ is a modal verb that describes what someone is able to do, i.e. general ability in the present. It is used in different forms to describe past and present abilities.

• 
  Peter can cook Italian food.
  

Positive statements

In positive statements we put ‘can‘ between the subject and the main verb in its base form:
subject + can + the verb (infinitive without ‘to’)

• 
  I can ride a bicycle.
  
• 
  They can help you with building your house.
  

Negative statements

The negative form of ‘can‘ becomes ‘can not’ or contracted ‘cannot’ (‘can’t’):

• 
  Jeffrey cannot play the piano.
  
• 
  She can’t go with us because she’s sick.
  

Questions

Questions with ‘can‘ are formed by swapping the subject and the modal verb:
Can + subject + the verb (infinitive without ‘to’)

• 
  Can Jane play tennis?
  
• 
  Can he drive a car?
  

Ability in the past

Could

To talk about ability in the past we use ‘could‘, which is the past form of ‘can‘.

• 
  I can’t dance rumba now (present ability), but I could when I was younger (past ability).
  
• 
  I could read when I was four.
  

The negative form of ‘could‘ is ‘could not (couldn’t)’. ‘Couldn’t‘ can be used to describe general and specific ability in the past:

• 
  I couldn’t go to that restaurant because it was too expensive. (general ability)
  
• 
  My grandfather couldn’t swim. (general ability)
  
• 
  He called us because he couldn’t find the house. (specific ability)
  
• 
  I couldn’t open the window. (specific ability)
  

Was able to

To talk about for specific ability in a particular situation in the past we can also use ‘was able to‘. The negative form of ‘was able to‘ is ‘wasn’t able to’ or ‘was unable to‘.
When the computer crashed yesterday, I was able to fix it. (not ‘I could fix it’).
She was able to pass the exam, even though she hadn’t studied much. (not ‘she could pass’)

Could have + Past Participle

To talk about an ability someone had in the past, but didn’t use, we can use ‘could have + Past Participle‘:

• 
  I could have passed the test well but I didn’t practise enough.
  
• 
  He could have come earlier.