Runes are used phonetically, so most of the time you should shorten double-letters to single-letters ("Hello" would become "helo"). All the appropriate runes are bound to the appropriate keys, so you can type freely without worrying about which rune you're using. However, a few runes which represent diphthongs which are unused in Modern English are bound to the SHIFT-number row. They are as follows: !-th, @-eo, #-ng, $-ɶ, %-æ, ^-ia/io, &-ea, *-kk. (-st. Additionally, in Old English, there are two types of "g"s, a soft "g" (which is bound to the "g" key), as in "sage", and a hard "g" (which is bound to the ")" key), as in "saga".
Keys 1-7 also include the different Roman numerals (I, V, X, L, C, D, M), which can be combined to make up a number (from what I can tell, the Anglo-Saxons probably used Roman numerals or tally marks - most likely the former).