"My turtle is growing slowly."
Translation:הצב שלי גדֵל לאט.
That seems to be the opposite of your first comment, and all I'm seeing at the moment is "growing slowly" and "גדל לאט", which would agree with your second comment anyway.
Could somebody tell me why we can not use "מְגַדֵּל" here? What's the difference between "גדֵל" y "מְגַדֵּל"?
Because it is a transitive verb - it has to act on an object. So for example "the farmer grows carrots".
I TRANSLATED : הצב שלי גדל באיטיות IT'S AN ADVERB AND IT PICTURES WELL THE SITUATION
You're right, we'll accept that. Next time please use the report function.
A few reasons, "the turtle" is הצב, and איטי (pay attention to the spelling) is "slow" rather than "slowly", which is לאט.
Often in colloquial Hebrew the adjective can be used instead of the adverb (הוא עושה את זה טוב, instead of הוא עושה את זה היטב he does it good vs he does it well), but in some cases it's more natural to use the adverb, like here with לאט.
Adding to the previous answer, in the "הצבי" you don't actually need the "ה" since it's implied from the possessive suffix י.
Please, correct me, if I'm wrong.