Just take a look at a list of words like though, through, although, thought, taught, taut, tough, throw, threw, thorough and you begin to get a sense of how difficult a language it is for learners. Ran across the anonymous poem below and thought it was a clever demonstration of the issues.
English Pronunciation Poem
I take it you already know
Of tough and bough and cough and dough?
Others may stumble, but not you
On hiccough, thorough, slough, and through.
Well don’t! And now you wish, perhaps,
To learn of less familiar traps.
Beware of heard, a dreadful word
That looks like beard but sounds like bird.
And dead: it’s said like bed, not bead,
For goodness sake don’t call it deed!
Watch out for meat and great and threat
(They rhyme with suite and straight and debt).
A moth is not a moth as in mother
Nor both as in bother, nor broth as in brother,
And here is not a match for there,
Nor dear and fear, for bear and pear.
And then there’s dose and rose and lose–
Just look them up–and goose and choose
And cork and work and card and ward
And font and front and word and sword
And do and go, then thwart and cart,
Come, come! I’ve hardly made a start.
A dreadful Language? Why man alive!
I learned to talk it when I was five.
And yet to write it, the more I tried,
I hadn’t learned it at fifty-five.
Mindship 
Great poem!