I'm going to use different sentences as examples, because "Have you decided to support our troops?" is a little bit of an odd question! "Have you decided to go to Oxford?" vs "Did you decide to go to Oxford?" "Have you decided to go to Oxford?" is the question you would ask if the person will be going to Oxford in the future. "Did you decide to go to Oxford?" is the question you would ask if going to Oxford happened in the past. If my friend in high school had been accepted to Oxford, I can ask "Have you decided to go to Oxford?" If I see a friend I haven't seen in a few years who I knew had been accepted to Oxford, I might ask them "Did you decide to go to Oxford?" Ultimately, you can use them interchangeably, but that's the difference in nuance. "Did you decide to support our troops" is an odd question, because your support of the troops would theoretically be ongoing.

Amy, If someone is asking me "Have you decided to support our troops?" Is the person asking me about certainty such as whether or not I'm determined to support their troops? Yes, that is possible, especially in American English. In that case the meaning would be conveyed by intonation. The word 'have' would be stressed in your sentence. HAVE you decided to support our troops? (Have you really decided?) The normal intonation of the sentence "Have you decided to SUPPORT our TROOPS?" is used to make the past event relevant to the present and carries the standard implication of the PRESENT perfect in American English. Does "Did you decide to support our troops?" mean I decided to support their troop in the past? -Yes, it does. But in American English that can also refer to the recent past, for example, five minutes ago. And does the sentence lack the any implied question about I'm supporting their troop still now? -If the time is mentioned, for example, "Did you decide to support our troops yesterday?" then it refers strictly to the past. When no specific time is mentioned as in "Did you decide to support our troops?" then it can be relevant to the present by context. The implication being --If I am asking about it now, then it is relevant now--. In everday speech the usage of the past simple without a time reference is very common in American English when referring to the recent past. In British English the present perfect is more often used to express the recent past. events. For example, Has you ever done an honest day's work in your life? British or American Did you ever do an honest day's work in your life? American

Good question! With the first sentence, it is implied that your answer of yes or no determines your certainty. The second sentence would imply that they have asked you whether or not you were going to support our troops but you were uncertain. Answering this question will remove the uncertainty the person assumed you had before. Good luck!