To the left of the letter area is a dynamic list of words you can choose. This will hopefully speed your entry of text.
Choosing 'M' will populate this list with words beginning with 'M'. Choosing 'A' will further refine the list to those beginning with 'MA'. This narrowing continues as you choose letters until there are no words that match. The word list initially begins with a set of the 1000 most common words in English. As you use a word more frequently it will eventually rise to the top of the list.
The above is called word completion.
There is also word _prediction_. A history of words that follow each other is maintained and used to predict what you might want to say next. Enter this:
and choose Clear. Then enter 'FROGS' again. The word 'EAT' will appear in the word list for you to choose, if you wish. If you do choose 'EAT' then the word 'BUGS' will appear. This list of 'following' words is presented in order of frequency just like word completion.
Note that word prediction only looks at the previous word. If you enter:
you will see BUGS in the list. Even though kids rarely eat bugs, frogs do.
In the upper left of the main screen there is an array of short words commonly used in English. These words are fixed and do not change like the word list.
Every time a Clear is done all the words in the message area are added to your word lists (_except_ for words that do not appear in the dictionary).
To delete a word from your lists, choose it, and then choose Cmd > DelW. The last word in the message area will be removed from your word lists (and from the message area).
If you wish a word to appear quicker in your word completion list choose it and then choose Cmd > TopW. The last word in the message area will now appear on the top of the list of words that begin with its first letter. It may not stay on top very long if it isn't often used. This technique can also be used for words not in the dictionary.