One of the greatest difficulties in working out the spelling of an English word on the basis of its sound concerns the unstressed syllables. Is it responsable or responsible, devide or divide? Often one cannot tell from listening to how the word is pronounced. In some cases the difficulty is compounded by interference: it is responsabel in Dutch and responsable in French but responsible in English; it is assistentin Dutch but assistant in English.
     This unclarity means that you will be wise to commit a number of highly frequent words to memory. Here is a list:

In addition to individual words, problems tend to lie with certain pairs of suffixes: -able vs -ible, -ant vs -ent, and -ary vs -ory vs -ery.