I saw a sentence” someone draw definite conclusion that…”, but conclusion is countable noun, so is it should be “someone draw a definite conclusion that…”. I searched both of them on Google, and found the former is bit more than the latter. I wonder if there are some differences between them.
Someone could draw a definite conclusion from the evidence shown by the attorney.
Someone drew a definite conclusion from the evidence.
Someone is drawing a definite conclusion with too little evidence.
Someone draws a conclusion, some other people do not.
Of course, I could be drawing rash conclusions.
But not "someone draw". The 'someone' is singular and therefore requires the verb in your example to have an 's' at the end.
Your question seems to be about the use of 'a' - using it is correct with only one conclusion.