I’ve answered the setIt/setAntecedents question, because it was quite easy question for me as I’ve used it in my own game. But in the first place, I know about these methods because prior to writing my game I’ve made a Czech translation of TADS and carefully replaced grammatical gender system by my own 
I can second the hint about fulltext searching the library, when you carefully choose keyword, there is chance you will pickup some explanation of the problem in comments.
Once you are quite familiar with Learning TADS 3 and all the coding excursus, you can take a look on the System manual (and Technical manual). Not to learn everything, but more to get some overview what is possible or at least what chapters are there. Later you should remember that you already saw something interesting there and you will know where to look.
For example find() is an intrinsic function of the String class and you could look here to have an idea what else is possible (comparing, finding, replacing, calculating length, matching, spliting, substringing, lower/uppercasing). Many of these features are easily anticipated because every programming language have something like this.
Oh, and by all means read the source code of Mike’s Return to Ditch Day, it a major sample game for TADS 3 and you will be amazed how many interesting tricks you will learn! setHim/getOrigText/rexSearch included
So there are lots of different ways, but in the end it is all about patiently gaining experience over time. I’m a professional programmer by day, I teach kids programming few hours a week, so it’s probably easier for me to gain the experience, but still I can get horibly stuck for months and yet there is someone who can answer with ease. So don’t be afraid to ask questions, good questions will lead to good answers which will be valuable resource for others. And give it the time, your second game will be much better and much easier to write then the first one.