"In reality, I am a turtle."
Translation:In werkelijkheid ben ik een schildpad.
It's more common to say "in werkelijkheid" than "in realiteit".
Also, word order in Dutch works differently. You need to have the verb come in the second position of the sentence.
The prepositional phrase "in werkelijkheid" comes in the first position in this particular sentence, so the first conjugated verb "ben" comes in the second position.
What if I use a comma to divide the sentence? "In werkelijkheid, ik ben een schildpad."
No, unfortunately that just doesn't work in Dutch.... The comma shouldn't be there, and the Subject+Verb-combo must be inverted into "ben ik" :) I believe that the rule is that in Dutch, the Verb Phrase must always(!) be the second phrase of the clause.