If someone asks if you are free at a certain time and invites you to an event, you are not required to commit yourself until you know what the invitation is for. For example, if you say that you’re free and the invitation turns out to be something you’d hate to do (a trip to a local sight you’ve seen ten times), then you will be in a position to take back your acceptance or invent an excuse later. If this happens, tell the person who invites you that you have to check and that you’ll tell him or her later. If an invitation must be refused, most people expect a reason. The following sequence would be appropriate: apology, reason for refusal, thanks for the invitation.
When someone asks you to his or her home, it is very clear who is the guest and who is the host, but invitations to restaurants for lunch, dinner, coffee, a drink, etc., sometimes present problems, and the customs vary in different parts of the United States.
In many instances it is the inviter who pays, as one would expect, but in some instances each one pays his or her own check: You “go Dutch.” This is often the case with friends in informal situations, such as “Let’s go and get a beer” or “Want a cup of coffee?” In some parts of the country, however, some people like to entertain friends by taking them to a restaurant for dinner instead of having dinner at home. In this case the host expects to pay and the guest may offer to leave the tip, which may be declined by the host. (If so, just let the matter drop.) If the invitation is expressed in fairly casual terms, such as “Let’s go to (name of a restaurant) for dinner,” it may be more of a suggestion than an invitation, so you should be prepared to pay your part of the bill.
If you want to invite someone for a meal at a restaurant, be explicit: “I’d like to take you to…”Americans should be explicit also, but they often assume you know the local customs in the matter. Ask a friend’s advice if you are not sure.
There are also phrases that sound like invitations but in fact are not. People may say things like: “We’ll have to get together sometime” or “You’ll have to come over and visit us sometime.” You can identify such non-invitations by their generality —— there is no specific time mentioned, and the word “sometime” is often used. They are often ritual expressions of parting. On these occasions you could respond: “Yes, that would be nice,” or “I’d love to”, or something similar, and then let the subject drop. If they do not call you, you could always invite them for some occasion.