。
I have to prepare for the English exam.
Literal
I [topic-は] English [possessive-の] exam [possessive-の] preparation [object-を] must-do-[なくてはならない].
~しなくてはならない — an obligation form built from ~なくては ('if not doing') + ならない ('it won't become'). Functionally equivalent to ~なければならない and ~なくてはいけない; the four variants combine the negative conditional (なければ / なくては) with the predicate (ならない / いけない) and are interchangeable in most contexts.