What is the difference between เคยค่ะ and ใช่ ?Feel free to just provide example sentences.

In Thai, ค่ะ/ครับ and ใช่ can mean the same thing.
They’re used to give an affirmative response and usually used together for politeness.
>> ใช่ค่ะ (female speakers)
>> ใช่ครับ (male speakers)

เคย indicates that the speaker has already done something before. When used as a response, it can be followed by ค่ะ/ครับ to sound more polite.
>> เคยค่ะ (female speakers)
>> เคยครับ (male speakers)

Responding to a question with “ใช่(ค่ะ/ครับ)” or “เคย(ค่ะ/ครับ)” depends on the question.

Examples;
1. “Have you ever been to Thailand?”
“yes, I have”
“คุณเคยไปประเทศไทยมั้ย?” “เคย(ค่ะ/ครับ)”

2. “You’ve been to Thailand before, haven’t you?”
“yes”
“คุณเคยไปประเทศไทยมาแล้วใช่มั้ย?”
“ใช่(ค่ะ/ครับ) or just simply “ค่ะ/ครับ”


*Notice that in the first example, the question requires that you answer with the auxiliary or modal from the question. That’s why you answer with “เคย(ค่ะ/ครับ)”

In the second example, the question only asks you to give an affirmative response or to agree with it. That’s why you answer the question with “ใช่(ค่ะ/ครับ)” or just “ค่ะ/ครับ”