Look up Etymology Instead of Spelling
Instead of looking up how to spell a word, I always look up the Etymology and find a link to another word I can spell. I don't know Latin or Greek, but just seeing the links between words often shows why people construct the spelling in this way or that.
Defence or defense?
It's 'defence', because it has the same root as a 'garden fence' (although both come from Latin 'defensus'
Which is 'tail', and which is 'tale'?
Tale comes from from Proto-Germanic 'talo' ('speech'), related to 'talk' and 'tell'.
The animal 'tail' comes from Old English 'tægl', like Gothic 'tagl', which gives us 'hair'. So think of 'tails have hair', and 'a tale is a talk with an "e"'.
'Cannal boat' or 'canal boat'?
'Canal' and 'cane' both emerged from Latin 'canalis', meaning 'water-pipe' or 'reed'.
Parralel, or parallel?
The 'para' there comes from the same place as 'paranormal'. So the prefix always comes with one 'r'.
Vicious or viscious?
The 'vic' part comes from 'ficere', meaning 'to do', same as 'maleficent', meaning 'bad-do'...or something. So think of 'viciousness is malice', and the single 'c' feels natural.
Supersede or supercede?
It means 'sitting above', or 'above-sit', as in 'super' + 'sedere' (same place we get 'sit' from). So think of 'super sit'.
'Compatible', or 'compatable'?
'Compatible' from Latin 'com' ('with') 'pati' ('suffering'), meaning 'to suffer together. This also gives us 'compassion', which also retains the 'i' as the third syllable.