Can,Could,Be able to, Be allowed to တို႔ကိုႏွိဳင္းယွဥ္ျကည့္ျခင္း (HZH)

Can vs Could vs Be able to vs Be allowed to

Can, could, be able to ကို ability (ကိုယ္လုပ္ႏိုင္တဲ့အရာတစ္ခုအေၾကာင္း ေျပာတဲ့အခါ) သံုးလို႔ရတယ္။

A. be able to ကို can/could အစားသံုးလို႔ရပါတယ္။ ဒါေပမဲ့ သူ႔ကို မသံုးရမယ့္ အေျခအေနေတြရွိတယ္။

👉 when we talk about something that is happening as we speak:
      ကၽြန္ေတာ္တို႔ေျပာသလို ျဖစ္ေနတဲ့အရာေတြအေၾကာင္း ေျပာတဲ့အခါ

e.g Watch me, Sister; I can stand on one leg.
     ❌ I am able to stand

👉 before passive
Passive ရဲ႕ေ႐ွ႕မွာလည္း သံုးလို႔မရဘူး။ ( be able to သံုးတာထက္ ပိုေကာင္းတယ္)

e.g CDs can now be copied easily.
❌ CDs are now able to be copied

👉 when the meaning 'know how to':
      တစ္ခုခုကို ဘယ္လိုလုပ္ရမယ္ဆိုတာ သိတယ္ဆိုတဲ့ အဓိပၸါယ္နဲ႔ေျပာတဲ့အခါ

e.g Can you cook?
    ❌ Are you able to cook?

B. If we talk about a single achievement, rather than a general ability in the past, we usually use 'be able to' rather than 'could'.
အတိတ္ကာလမွာ တစ္ခုခုလုပ္ႏိုင္တယ္ဆိုတာကို general ability ဆိုတာထက္ specific achievement ကို ေျပာရင္ could ထက္ be able to ကို သံုးရတယ္။

She could play the guitar well and she was able to play "Perfect" sung by Ed Sheeran.
သူ(မ) ဂစ္တာတီးတတ္တယ္၊ (general ability)
Ed Sheeran ဆိုတဲ့ perfect သီခ်င္းကို ေကာင္းေကာင္းတီးတတ္တယ္။

He could swim strongly and was able to cross the river easily, even though it was swollen by the heavy rain.

🚩ဒါေပမဲ့ be able to ထက္ could က ပိုသဘာဝက်တဲ့ အေျခအေနေတြ႐ွိတယ္။

👉 in negative sentences  (အျငင္းဝါက်ေတြမွာ)

I tried to get up but I couldn't move. (rather than I wasn't able to move)

👉 with verbs of the senses - e.g. feel, hear, see, smell, taste and with verbs of thinking - e.g. believe, decide, remember, understand
sense နဲ႔ဆိုင္တဲ့ verb ေတြ၊ thinking နဲ႔ဆိုင္တဲ့ verb ေတြဆိုရင္ could ကို သံုးေပးရတယ္။

I could remember the crash, but nothing after that.

👉 after the phrases the only thing/place/time  and after 'all' when it means 'the only thing'.
The only thing/place/time စတဲ့ phrase ေတြနဲ႔ all နဲ႔စၿပီး the only thing လို႔ အဓိပၸါယ္ရတဲ့ phrase ေတြမွာ could ကို သံုးရတယ္။

The only thing I could tell you back is that I was unlucky.
All we could see were his feet.

👉 to suggest that something almost didn't happen, particularly with almost, hardly, just, nearly:
almost, hardly, just, nearly စတဲ့ adverb ေတြနဲ႔ တစ္ခုခုဟာမလုပ္ႏိုင္/မျဖစ္ႏိုင္ဖို႔ ျဖစ္လုနီးပါးဆိုတာကို ေျပာဖို႔။

The air was so polluted in the city centre, I could hardly breathe.

Can & Could talking about possibility 

To talk about theoretical possibility of something happening we use 'could', not 'can'. However, we use 'can', not 'could' to say that something is possible and actually happens.
သေဘာတရားအရျဖစ္ႏိုင္တယ္ဆိုတာေျပာဖို႔ could ကို သံုးရတယ္၊ ❌ Can သံုးလို႔မရပါဘူး။
တကယ္လက္ေတြ႕ျဖစ္ႏိုင္တာကိုေျပာဖို႔ကိုေတာ့ can ကို သံုးရပါတယ္။ ❌ Could မသံုးရပါ။

Could 👉 theoretically possible
Can 👉 Actually Possible

It could be expensive to keep a cat.
ေၾကာင္တစ္ေကာင္ေမြးရတာ စရိတ္စကမ်ားႏိုင္တယ္ (တကယ္မေမြးရေသး၊ ျဖစ္ႏိုင္ေခ်ရွိတာေျပာတာ)
It can be expensive to keep a cat.
ေၾကာင္တစ္ေကာင္ေမြးရတာ စရိတ္စက မ်ားတယ္။

✏ We use 'can't', not 'couldn't', to say that something is theoretically and actually impossible.
      theoretically ေရာ၊ actually ပါ မျဖစ္ႏိုင္ဘူးလို႔ေျပာဖို႔ can't ကို သံုးရပါတယ္၊ couldn't မသံုးရပါ။

There can't be many people in the world who haven't watched television.

✏ We use 'can' to indicate that there is a very real possibility of a future event happening. Using 'could' suggests that something is less likely or that there is some doubt about it.
အနာဂတ္အျဖစ္အပ်က္တစ္ခုဟာ ျဖစ္ႏိုင္ေခ်မ်ားတယ္ဆိုရင္ can ကို သံုးရပါတယ္၊ ျဖစ္ႏိုင္ေခ်နည္းတယ္ဆိုရင္ေတာ့ could ကို သံုးရပါတယ္။
We can stay with John in New York. (= We will be able to stay)
We could stay with John in New York. (It's possible if he's there)

🚩 Could vs be allowed to : talking about permission 

✏ To say that in the past someone had general permission to do something — that is, to do it
      at any time — we can use either 'could' or 'was/were allowed to'. However, to talk about
permission for one particular past action, we 'was/were allowed to', but not 'could'.
အတိတ္ကာလမွာ တစ္ခုခုလုပ္ဖို႔ခြင့္ရတယ္ (လုပ္ခြင့္ရတယ္ဆိုရင္) could နဲ႔ was/were allowed to ႏွစ္ခုလံုး သံုးလို႔
ရပါတယ္။ ဒါေပမဲ့ action တစ္ခုကို အေလးေပးေျပာရင္ေတာ့ was/were allowed to ကိုပဲသံုးရတယ္၊ ❌ could
သံုးလို႔မရပါ။

Anyone could/was allowed to fish in the lake when the council owned it.
ေကာင္စီက ကန္ကိုပိုင္တုန္းက အဲ့ကန္မွာ ဘယ္သူမဆိုငါးဖမ္းလို႔ရတယ္၊ (ဘယ္သူမဆိုရတယ္;ထူးထူးျခားျခားမဟုတ္ဘူး)

Although he didn't have a ticket, Ken was allowed to (❌could) come in.
သူ႔မွာ လက္မွတ္မရွိေပမဲ့ ကမ္က အထဲဝင္လို႔ရတယ္။

✏ In negative sentences, we can use either 'couldn't' or 'wasn't/weren't allowed to' to say
that permission was not given in general or particular situations.
အျငင္းဝါက်ေတြမွာေတာ့ ခြင့္မရဘူးဆိုတာကို ေျပာဖို႔ general ပဲျဖစ္ျဖစ္၊ particular ပဲျဖစ္ျဖစ္ couldn't နဲ႔
wasn't/weren't allowed to ႏွစ္ခုလံုးသံုးလို႔ရပါတယ္။

We couldn't/weren't allowed to open the presents until Christmas morning.

Thanks
HZH

Ref:

Unit 15,Advanced Grammar in Use; Cambridge University Press