The word "perhaps" and "possibly" are adverbs, while "could be" is a verb made of the auxiliary verb "could" and the main verb "to be". Because they are different parts of speech, the first two are NOT interchangeable with the third. And, in some cases, only one of the three will work. Example: "Perhaps you shouldn't drive until you've slept." This is a polite suggestion, which requires "perhaps" or "maybe". Let's try it with the other two: "Possibly you shouldn't drive until you've slept." This doesn't work. "Could be you shouldn't drive until you've slept." No, this doesn't work, either. In other situations, either "perhaps" or "possibly" will work. Example: "He is perhaps the tallest man in our state." "My friend Tom is possibly the tallest man in our state." However, let's see how "could be" works here: "My friend Tom is could be the tallest man in our state." This doesn't work, because "could be" is a verb, and we cannot have two verbs ("is" and "could be"). We would have to remove the verb and replace it with "could be". "My friend Tom could be the tallest man in our state." Notice, here, there is a question as to the meaning. It could be understood as "It is possible, but I'm not sure." Or it could be understood as "If circumstances change, he WILL be." That is, for instance, IF the tallest man in the state moves or dies, then Tom WILL BE the tallest man in our state. The word "perhaps" has the most formal mood of the three.

HI Johnny. Perhaps and Possibly are synonyms. They mean the same thing. The definition of "perhaps" is: maybe; possibly. "Could" is an adverb with simmilar meaning, but the phrase "could be" means the same thing as "perhaps".