I don't know
I dunno
/aɪ dəˈnoʊ/ReductionElision
In real speech
What happens to the sounds
The t in don't drops out and the vowel weakens to /ə/: don't know → “dunno”.
When to use it
Casual conversation. It can sound careless, so in a job interview say I'm not sure.
Casual → formal
Slang
dunno
Neutral
I don't know
Formal
I'm not certain
Sound notes
Reduction: unstressed words get squeezed, and their vowels turn into the neutral sound /ə/.