While you are giving evidence, the lawyers and the judge may object to certain things that you say. This is because there are legal rules relating to what evidence you may give.
The judge may interrupt you and ask you to stop if one of the rules of evidence is being broken.
As a witness, you may only give evidence about facts within your own personal knowledge. You cannot testify as to what somebody else saw or heard.
You may not give evidence about your opinion. You must confine your testimony to a statement of the facts, not any assumptions you have made or conclusions you have drawn from those facts. The exception to this rule is the expert witness who may give evidence as to his or her opinion.
You may not give evidence about what someone else said. This is known as the hearsay rule. For example, you may not give evidence that your mother told you she saw the accused grab your purse. In that case your mother must herself give evidence in court that she saw the accused grab your purse.
You may not give evidence about previous statements you have made. For example, you cannot give evidence that you told your friend that the accused grabbed your purse. You may only give evidence about the actual incident. There is one exception to this rule; if you are giving evidence as a victim of rape, you are allowed to introduce evidence that you made a complaint after the rape.
Your main duty as a witness is to tell the truth to the court. If you knowingly make a false statement under oath, you may have committed the crime of "perjury" and you may be prosecuted.