In many cases these forms are fixed arbitrarily, and in some there is even now disagreement among the highest authorities.
These difficulties and disagreements have two reasons: First, English is a composite language, drawn from many sources and at many periods; hence purely philological and etymological influences intervene, sometimes with marked results, while there is a difference of opinion as to how far these influences ought to prevail.

Second, the English language uses an alphabet which fits it very badly.

Many letters have to do duty for the expression of several sounds, and sometimes several of them have nearly or quite the same sound.

For example, there are a number of distinct sounds of _a_, _i_, and _o_ while _g_ is sometimes indistinguishable from _j_ and _c_ from _k_.
This is not always a matter of modification of sounds by the sounds of other letters combined with them.

One has to learn how to pronounce _cough_, _dough_, _enough_, and _plough_, the _ough_ having four distinct sounds in these four words.