Каждый модальных глаголов – can/could/may/might/must – обладает своей степенью предположения и уверенности по сравнению с остальными.
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How is the verb tense modal verb «could» used?
The English verb «could» is used as both an auxiliary verb and a modal verb? Could is almost always used with a main verb tenses.
- Could is used as the past tense of «can».
- Could is used to is used to request something in the present tense.
- Usually when could is used in the present tense, it is used to ask a question.
- Could the girls come with me to the store?
- I could not go to the moves last night, I had to study.
- We couldn’t help you clean the house.
Structure of «could»:
- When using as a modal verb or an auxiliary verb the verb «could» stays in its base form.
- Could does not change form according to the subject.
- Could is followed by a main verb. The main verb also stays in its base form. (The main verb that is used with could doesn’t change form according to the subject.)
|Positive Sentences||I||could||—||go||to the store|
|Negative Sentences||We||could||not||go||to the store|
|Questions**||Could||they||—||go||to the store|
* All subjects are: I, we, they, you, plural nouns, singular nouns and uncountable nouns.
**Questions — When could is used to ask a question, could comes before the subject.
— See more at: http://english-the-easy-way.com/English_ESL/Could_English_Verb.htm#sthash.s9hrMinK.dpuf