Assuming it’s a noun, these words might be used as shown below, though there are other meanings.)

A deal is a good value. (I got a good deal on that car.)

A treat is something very enjoyable and maybe unusual. (Meeting that musician and listening to his music was a real treat.)

The two words, as verbs, can be quite similar. The following sentences mean almost exactly the same thing:

“How shall we deal with this problem?”

“How shall we handle this problem?”

There IS a subtle difference, however.

Imagine that there is a problem. If it’s a routine problem, then you would “handle” it.

If the problem is bigger, and if it’s gotten to be a HUGE problem, then you might “deal” with it.

So the SIZE of the problem could determine which to use.

However, to tell you the truth, this distinction is very subtle and I wouldn’t worry about it. A CAREFUL and EXACT writer might understand the distinction I’m drawing here, but the average EDUCATED American would use the two words interchangeably.

Bottom line: the difference between the two is unimportant.

These two words have no similarity in meaning.

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