do not know or don't know

[emsr2d2]

Not often, in BrE at least. My late grandfather used "I mightn't ..." sometimes but we don't hear it very often now. I use "I might not ..." in both spoken and written English.

 

[GoesStation]

Do you contract "might not" in speech?

Not in American English.

 

[andrewg927]

Same goes with "may not".

 

[GeneD]

What about "he/she/it was", "you/we/they were", "there was/were"? Are they contracted in speech somehow?

 

[GoesStation]

In my dialect the vowel in was can become extremely short when it's not stressed but it never disappears completely. Listeners unused to our accent might not hear it in its shortest incarnation.

 

[emsr2d2]

What about "he/she/it was", "you/we/they were", "there was/were"? Are they contracted in speech somehow?

In short, no. Let's take "There is" as an example. You will see that the past simple is the only tense that can't be contracted:

There is = there's
There has been = There's been
There had been = There'd been
There will be = There'll be
There will have been = There'll have been (or even "There'll've been"!)

There was = There was (that's it)