By and With
Incorrect : I cut the mango by a knife.
Correct : I cut the mango with a knife.
Incorrect : The rat was killed with a cat.
Correct : The rat was killed by a cat.
Note: When you mention an instrument with which something is done, you should use “with". On the other hand, 'by' indicates the doer or performer of an action. But there are exceptions to this general rule. For example:
I wrote the letter by hand. (not “with")
I sent the message by telephone. (not "with")
He is reading by the candle light. (not “with")
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