There are 9 parts of speech or word classes in English language:
– nouns
– pronouns
– determiners
– verbs
– prepositions
– adjectives
– adverbs
– conjunctions
– interjections

There are 2 types of verbs:

– main verbs
(have their own meaning, can stand on their own)
Examples: walk, work, study, play, sing, etc.
I walk to school every day.

– auxiliary verbs (helping verbs)
(cannot stand on their own, needed to complete a sentence)
Examples: is,am,are,do,does,did,have,has,had
I am walking to school now.

Modal verbs:
– are part of auxiliary verbs
– cannot stand on their own, must be used with main verbs

10 modal verbs:
– can
– could
– will
– would
– shall
– should
– may
– might
– must
– ought to

1. Present or future ability to do something
– Jane can speak French fluently.
– I can’t hear you.
– We can go to the park tomorrow.

2. Possibilities
– Smoking can cause cancer.
– The application process can take a week.
– You can’t always get what you want in life.

3. Requests or orders
– Can you turn on the TV?
– Can you come here a minute?
– Can you be quiet?

4. Asking for or giving permissions
– Can I smoke in this room?
– You can’t smoke here.
– You can smoke in the garden.

5. Offers to do something
– Can I help you?
– Can we do anything for you?
– I can help you with the report if you want.

(+) Subject + can + base form
I can play tennis.

(-) Subject + cannot + base form
(-) Subject + can’t + base form
I cannot play tennis.
I can’t play tennis

(?) Can + Subject + base form
Can you play tennis?

‘Can’ can only be used with base forms.
I can play tennis.
I can to play tennis. (wrong)
I can played tennis. (wrong)
He can plays tennis. (wrong)
We can playing tennis. (wrong)

‘Cannot’ is written as one word, not as two words.
I cannot swim.
I can not swim. (wrong)

Non-action sentences:
– descriptions
You can be a pilot one day.
– conditions
You can be happy if you want to.
– locations
We can be there tomorrow.

Descriptions / professions are nouns:
– student
– teacher
– doctor
– father
– mother
– friends
– cousin
– etc.

Conditions/ qualities/characteristics/emotions are adjectives:
– tall / short
– happy / sad / fast / slow
– beautiful / ugly/ wonderful / terrible
– old / young / new / cheap / expensive
– kind / laid back / ambitious / passionate
– etc.

Locations are adverbs of place:
– here
– there
– home
– inside / outside
– upstairs / downstairs
– etc.

(+) Subject + can + be + Noun / Adjective / Adverb of Place
You can be a doctor.

(-) Subject + cannot + be + Noun / Adjective / Adverb of Place
(-) Subject + can’t + be + Noun / Adjective / Adverb of Place
You cannot be here.
You can’t be here.

(?) Can + Subject + be + Noun / Adjective / Adverb of Place
Can you be quiet?

1. Past ability
– I could swim when I was 5 years old.
– When we got home last night, we could not open the door.
– My late grandmother could speak seven languages.

2. Present/future possibilities
– The storm could get worse.
– His recovery could take months.
– Anything could happen in the next half an hour

3. Present or future suggestions
– I could look after the children for you.
– We could have lunch one day.
– You could call Susie and ask for her help.

4. Polite request/permissions
– Could I speak to Mr. Davis, please?
– Excuse me, could I just say something?
– Could you open the door, please?

(+) Subject + could + base form
I could play tennis.

(-) Subject + could not + base form
(-) Subject + couldn’t + base form
I could not play tennis.
I couldn’t play tennis

(?) Could + Subject + base form
Could you open the window?

‘Could’ can only be used with base forms.
I could play tennis.
I could to play tennis. (wrong)
I could played tennis. (wrong)
He could plays tennis. (wrong)
We could playing tennis. (wrong)

‘Could not’ is written separately, not as one word.
I could not swim.
I couldnot swim. (wrong)

(+) Subject + could + be + Noun / Adjective / Adverb of Place
I could be wrong.

(-) Subject + could not + be + Noun / Adjective / Adverb of Place
(-) Subject + couldn’t + be + Noun / Adjective / Adverb of Place
He could not be there.
He couldn’t be there.

(?) Could + Subject + be + Noun / Adjective / Adverb of Place
Could he be an actor?