To be forgetful is to be absentminded. When you're forgetful, things tend to slip your mind. People can be forgetful if they really can’t remember things, or if they’re just not paying attention.
When you're forgetful, you forget all kinds of things! A forgetful person misses deadlines and forgets about their spouse's anniversary. Forgetful people are often spacy and careless; they're not mindful of what they should be doing. A forgetful gardener might forget to water the plants and go for a walk instead. If you’re late, you’re forgetful of the time. If you’re forgetful, don’t get a pet. Even goldfish need attention!