Ever thought about the ends of these words?
'-tain' may not be as simple as you think.
What about 'captain', 'mountain' and 'fountain'?
​They are different to 'certain' and 'curtain'.
Which are different again to words like 'contain' and 'sustain'.
You see?
And are you using your own language as a reference?
It's what a Turkish client of mine was doing. The word 'captain' in Turkish is 'captan'. She was using the pronunciation of the vowels in the Turkish word in the English word - which seldom works!
Watch the video above to correct your pronunciation of '-tain' word ending.