If my boyfriend is accused of committing a crime and I was present, since I was subpoenaed to testify against him can I plead the 5th?

You can always invoke you 5th Amemdment Right against self-incrimination and refuse to answer any question or testify under oath if you believe your answer will indeed incriminate you. If you are testifying at a trial, the judge can have you speak with a attorney to determine if your answer will indeed incriminate you, but you can always invoke your 5th Amendment right.