I am an F1 student on a leave of absence from my PhD program. I want to attend a 5-day conference in the USA. Can I enter using my valid B1/B2 visa? This entry will only be for the conference; I will re-enter using my F1 visa once I get my new I-20.
Yes, you can enter with a valid B1/B2 visa, as a visitor and to attend a short conference. You may want to have a copy of the I-94, so that your university is able to verify the time you were in the US, and that you did not enter under the F-1, and so that it doesn't conflict with the issuance of the new I-20.