Unit 5 in the small Anniversary manual, the lesson on this board here and on Angelfishy here explains this very well.
Basically, in Anniversary there are two ways to write the letter S. This is to simplify the writing and to create fluid in writing. Having one way to write the letter 's' can work (does in later editions), but with the two ways to write a letter 's' you get more speed.
The way you write the 's' will depend on a few factors. Mostly the way the remainder of the outline will be written. No you don't need to memorize every 's' sounding outline in the language. You will learn this as you practice.
Practice the drills to become an expert on the 's' outline.
Teaching by the Analytical Method starts on page 112 here, and goes through page 122, has more details about how to write the 's' and determine how to write it. This is great for those who want more instruction.
The discussion on this book on this board is here.
The book suggests looking at the following two words and comparing them. It shows a great example of the different 's' directions that are used to aid in writing, reading and transcribing.
Even if a brief form, abbreviation, etc,. uses one 's', other words may switch it up for ease of writing. If you are ever in doubt, write it any way you want, then look it up when you have a chance.
You will notice that "countryside' is actually two shortened words combined. Both are brief forms. Because "country" is a brief form and starts the word, you keep that direction for the 's' brief form (note that a lot of these combined words are often flexible in the rules and your own personal writing as well).
If you'd like more practice on the variations of the outline 'side' you can find them here greggshorthand.proboards.com/post/7880 . These will give you help in seeing how a brief form and the left and right 's' outlines are used.
The word "still" has the same direction for the letter "s". However, different words will give you practice in writing this. I found this by finding the word "instill" under the "I" (I guess for the 'in' word beginning) and the others under the "still" word). There might be others but these are good practice.
You will note, however, that the word ending 'ness' uses the other direction for the 's'.
"Gregg Shorthand had achieved the most extraordinary success ever attained by any system in the history of shorthand." John Robert Gregg, 1922