When you begin a sentence with the word unless, you are writing a subordinate clause. You will need to add an independent clause to complete your complex sentence.
Until you have decided on a color
Until we rake the lawn
Until the bell rings
Each of these subordinate clauses is an incomplete sentence. We must add an independent clause, like this:
Until you have decided on a color, we can’t buy any paint.
Until we rake the leaves, they will continue to pile up against the fence.
Until the bell rings, we have to stay in the classroom.
The word unless is also a subordinate clause word. If you begin a sentence with the word unless, you must add an independent clause to make a complete complex sentence.
Unless you have something else to say, I think the conversation is over.
Unless you’ve got an extra ten dollars, we can’t afford to buy this.
Unless Bob can get off work early, we won’t have enough players for the game tonight.
Whenever you begin a sentence with the subordinate clause words until or unless, add a comma and an independent clause to make a complete sentence.