Do AMericans pronounce " forty" as fordi or forti?

Usually "fordi"

Mr is "Mister"

The T will often become a D after an R or between vowels, which are voiced, although not if it comes at the end of the word.

Congradulations. Petals (on flowers) will sound like pedals (on a bicycle).

Mister - the T comes after an unvoiced S. (You also left out the S in your suggested pronunciation.)

I don't know, Barb. I have trouble accepting the idea that the T in congratulations is pronounced like a D. To me the TU part of that word ends up sounding more like the word 'chew'.

Yankeesounding more like the word 'chew'.

I do that one with the CH sound, so what you say sounds reasonable.
Also, I do:
twenny, thirdy, fordy, fifty, sixty, sevendy, aidy, ninedy
CJ

There's no real D in "congratulations", but I'm sure GG meant to say she pronounces it as "congra-jew-lations", with JEW instead of CHEW.
And Americans say FORDY, yes. Not "misder".

But American Accent Course (Ann Cook) says: The soft T is pronounced like a d, so why mister here?
Is the same true with doctor?

Read better what Ann Cook said. I think I remember reading she says "T is D in the middle of a staircase", since she always mentions "staircases" for intonation patterns.
Mister and Doctor don't have a tapped T (=a "D" sound).

But I believe mister and doctor both stand at the middle of the staircase just as betty, better etc. ?

t is not pronounced d after an s or a c and in many other situations. A vowel, including r-colored vowels, must precede and a vowel must follow, and the stress must be in the syllable before the t.
See How to pronounce a 't' in Ameri

My students always have questions about this pronunciation feature. I know of four patterns that might help you predict where to pronounce 't' as a soft /d/.

1. when a ‘t’ is between vowels when the preceding vowel is stressed in words such as: city, party, forty, water

2. when a ‘t’ comes before a syllabic /l/ sound in words such as: petal, metal, settle

3. in past ten