I was thinking about this in the context of conversations I have with my 2 year old. I try to be careful - sometimes it the morning she'll ask if we can go to the park, and I say "_Maybe_ we can go to the park" if I'm not sure we can fit it into our schedule.
However sometimes I'm wrong. She'll ask "Read book?" and I'll say, "We'll go downstairs and read a book" and then we go downstairs and distractions happen - her brother needs breakfast, the kitchen's a mess, other stuff comes up. I may follow the letter of the law by reading her a book an hour later, but it'd be fair of her to feel a little put off. I do try to avoid doing that, but it's not always possible.
However I reserver the words "I promise" for times when I am ( almost ) 100% sure I won't break my word to her. She had to have eyedrops recently at night, and now at bedtime she'll ask "No eye drops" and I say "No eye drops, I promise". I'm trying to give her some security by being very specific about the times when I know something is or isn't going to happen.
( She also often worries about going to the doctor's. "No doctor?" she'll say, concerned, if we go out somewhere. "No doctor, I promise" I reassure her. I sometimes worry that one day we're going to get into an accident at the store, and we'll end up at the doctor's, and she'll never trust me again. )
- new post arrived, now I'm no longer being responded to? It was, truly, a genuine question.