Predictor
As you write in a Document, the Predictor can suggest words that:
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Begin with letters you've typed e.g. typing 'wal' suggests words like 'walk' / 'walked' / 'walking'
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Begin with the phonetic sounds of letters you've typed e.g. typing 'sor' suggests words like 'saw' / 'source' / 'sword' alongside words like 'sort' / 'sorry'
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Often follow the previous word e.g. typing 'this' and then space suggests words like 'is' / 'was' / 'is a'
To turn the Predictor on/off, click 
in the toolbar.
Suggestions appear either in a floating window that follows the text cursor, or above the On-screen Keyboard/current Clicker Set.
Click a suggestion to use it. To hear it spoken first, right-click it (or use Sound Shift).
Number Keys
To turn number keys on/off, click 123 in the floating window's bottom bar.
When enabled, pressing a number key on your device's keyboard will select the corresponding suggestion.
Customise
You can customise how suggestions are displayed, along with the levels of vocabulary and phonetic matching used.
To do this, see Suggestions Settings.