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Jesus continues to teach his disciples. This event began in Matthew 5:1. Jesus is talking to a group of people about what might happen to them as individuals. "You have heard" and "I say to you" are plural. "You will not" is singular, but you may have to translate it as plural.
commit
This word means acting out or doing something.
But I say to you
The "I" is emphatic. This indicates that what Jesus says is equally important to the original commands from God. Try to translate this phrase in a way that shows that emphasis, as in Matthew 5:21.
everyone who looks on a woman to lust after her has already committed adultery with her in his heart
This metaphor indicates that a man who lusts after a woman is as guilty of adultery as a man who actually commits the act of adultery. (See: rc://en/ta/man/translate/figs-metaphor, rc://en/ta/man/translate/figs-metonymy)
looks on a woman to lust
"desires in the mind to have another woman"