When does the soul enter the body. We know that but when a person dies the soul goes back to God .

The image of a soul either entering or leaving the body is a visual parable representing something deeper. The soul, not being physical, does not take up space. The connection of the soul to the body does not totally end with the death of the person (see Gesher HaChayyim at length) and even determining the exact time of death raises halachic questions. From the time of conception until forty days after conception, the embryo is considered not fully formed and this has certain halachic ramifications (Nishmat Avraham Choshen Mishpat 425a). The connection of the soul with the body reaches various stages of development after that time, but this is an esoteric issue.