Calvinists insist that Penal Substitution is proven by the fact the Bible often says that Jesus "died for" us (e.g. 1 Cor 15:3), thinking that this means that Jesus 'took our place' in God's divine 'electric chair'. While that claim is understandable, that is not automatically what we should assume, since to do something "for" another commonly just means "on their behalf," not necessarily in their place. For example, to "pray for" your enemy (Mt 5:44) does not mean you prayed what they were supposed to pray in their place. Rather, it just means you prayed on their behalf (cf Acts 12:5).
When I looked up the term "for" in Greek, of the 170 times it was used it most often meant something along the lines of "on behalf of," and rarely did it mean "in substitution of" another person.