How is it likely to get pregnant with a condom on but is not possible to get an STD with a condom on?

It is not likely that pregnancy will occur when using a condom. Condoms prevent pregnancy 96 - 98% of the time.

Pregnancy can occur on some occasions when using a condom if the condom has a small hole and semen leaks into the vagina. Or, more severely, if the condom splits completely.

STDs are transferred by the exchange of bodily fluid, so when vaginal secretion enters the hole on the head of the penis or is absorbed by the head of the penis, or when semen is deposited in the vagina and absorbed in the vaginal walls into the blood. The penis can leak semen before ejaculation, so ejaculation does not have to occur in order to transfer an STD from male to female.

So, the probability of pregnancy occurring whilst using a condom is less than the probability of transferring an STD. This is because if the condom fails, the STD will probably be transferred. But, even if semen is deposited in the vagina, pregnancy is not certain, as this can depend on the stage of the female in her menstrual cycle.