substitute for (someone or something)
1. To act as a substitute for someone or something. I'm substituting for Eric for the next few days while he recovers in the hospital. Don't believe anyone who tries to convince you that low-fat cooking sprays can substitute for real butter or olive oil and not taste any different.
2. To use, employ, or instruct someone or something to act as a substitute for someone or something else. In this usage, a noun or pronoun is used between "sub" and "for." With a healthy 30-point lead, the coach substituted the rookie for the star quarter back to give him some experience on the field. Let's try substituting an AC motor for the DC one we've been using and see if that improves the results.
substitute someone for (someone else)and substitute something for something else
to exchange someone or something for someone or something else; to replace someone or something with someone or something else. Shall I substitute Fred for Mary in the front office? Please substitute fish for beef on my dinner order.
substitute for someone or something
to serve as a replacement for someone or something. I have to substitute for Roger at work this weekend. Do you think that this will substitute for the one you wanted?