it should be:
وتحولت الصخرة الى  ضفدع   
as تحولت  is female form so we must put female noun  الصخرة   instead of the male noun الصخر

you read it: tahaw walat

Hi Ivan,

you are right. It's the V. form  "tahawala" .=  turned to 

"tuhawala" doesn't exist. 

But what is the form of the verb ? 
the form is تفعّل ( I'm not sure to get your question)

it's in past.
The rock turned to a frog.

and what does mean then tuhawwalat ? with "u"
write it in Arabic letters so I can know what's this word.

Present
I change: Atahawal
you change: tatahawal

Past
I changed: Tahawaltu
You changed: Tahawalta

watahawwalat 

The rock turned into a frog

Yes, I made a misstake it was  الصخرة

But what is the form of the verb ?
and what does mean then tuhawwalat ? with "u"

how you I changed ?
tahawwalatu ?
or I change (now) ?
atahawwal ?