You can get pregnant any time you have sex without a family planning method (even if it is only once).
You can get pregnant even if the man thinks he did not let his seed (sperm) come out.
You can get an STI or HIV if you do not use a condom when you have sex with an infected person. And you cannot tell by looking at a person if he is infected.
It is easier for a girl to get a sexually transmitted infection (STI) or HIV from a boy or man than it is for her to give these diseases to him. This is because of the way sex works — because she is the ‘receiver’. It is also harder to know if a girl has an infection, because it is inside her body.
Always use a condom for protection against STIs and HIV. But the most certain way to avoid pregnancy, STIs, and HIV is to not have sex.
Sources
Burns, A. A., Niemann, S., Lovich, R., Maxwell, J., & Shapiro, K. (2014). Where women have no doctor: A health guide for women. Hesperian Foundation.