Could का प्रयोग Can  के Past Tense Form  के रूप में किया जाता हे इसलिए indirect narration में की जगह किया जाता हे 

He said ,"I can teach English '
He said that he could teach English

To express past ability(भूतकाल में निहित योग्यता बताने के लिए किया जाता )

He could get a job. = He was able to get a job.

To express a polite request ( विन्रम अनुरोध करने के लिए )

Can I come in ?
Could I come in ?

 To express condition (शर्त की अभिव्यक्ति करने के लिय) 

If he were a minister , he could buy an aeroplane

To express ability in the present or future(वर्तमान या भविष्य कालीन योग्यता बताने के लिए किया जाता)

You could of course do well if you study regularly
You could solve this problem in on time

To express suggestion (सलाह की अभिव्यक्ति करने के लिय)

You could talk to the chairman in this regard
Some  phrasal used of could
I could't help going there

To express possibility in the past(मुहावरेदार प्रयोग करने के लिए)

You could have done the work alone