Could anyone help to tell the difference between these two sentences: "That's the thing I want to do" and "That's the one thing I want to do" Thanks
Yes, there is a very subtle difference.
In the first sentence-
"That's the thing I want to do"
You are implying to do a(that) thing from among other possible choices that you have. For instance- if you're having a family conversation about a picnic, and various options are being considered; someone says out loud -"Let's go for a hike". And, since you like hiking, you say- "Yes, that's the thing I want to do".
Now, coming to your second sentence-
"That's the one thing I want to do"
Here, "the one thing" is comparable to do something that you have always wanted to do. It is sort of a very importable objective in your life, something like one's Bucket List
So, you can use "that's the one thing I want to do" as-
"Climbing the Mt. Everest: That's the one thing I want to do before I turn 40".
Take a look at this example of "The one thing" from this Forbes link
The One Thing Successful People Do Every Day
We’re all busy. We all have infinite to-do lists that fill up faster than we can cross off the most urgent tasks. Being busy makes you feel productive, but it could be getting in the way of your most important project: YOU.
“Busy” is the most common reason people give me for not doing anything to build their brand so they can advance their career. They make time for emails and meetings and teleconferences, but they don’t capture the true benefits of all those activities. Working in their career is getting in the way of working on their career. Sound familiar?
Well, here’s the one personal branding habit you can’t be too busy for.
Document your wins. What’s the easiest way to do that? Keep a job journal.