Sforno on Genesis 38:11/ספורנו על בראשית ל״ח:י״א

1שבי אלמנה, wait for a while in a state of widowhood. This expression also occurs in Hoseah 3,3 ימים רבים תשבי לי, “you will have to go a long time without marrying.”

2פן ימות הוא כאחיו, so he would not be so preoccupied with Tamar’s beauty due to his immaturity which had deflected his brothers from the purpose of marriage, and he too would die as a result of this.