Your students will have to study the pronunciation "on a case-by-case basis".
There is no logic to explain why read is /ri:d/ in the present form and read /red/ in the past form.
The same applies to many other cases.
When the English tongue we speak.
Why is break not rhymed with freak?
Will you tell me why it �s true
We say sew but likewise few?
And the maker of the verse,
Cannot rhyme his horse with worse?
Beard is not the same as heard
Cord is different from word.
Cow is cow but low is low
Shoe is never rhymed with foe.
Think of hose, dose,and lose
And think of goose and yet with choose
Think of comb, tomb and bomb,
Doll and roll or home and some.
Since pay is rhymed with say
Why not paid with said I pray?
Think of blood, food and good.
Mould is not pronounced like could.
Wherefore done, but gone and lone -
Is there any reason known?
To sum up all, it seems to me
Sound and letters don �t agree.